package storage

import (
	"bytes"
	"container/heap"
	"errors"
	"fmt"
	"io"
	"path/filepath"
	"sort"
	"strconv"
	"sync"
	"sync/atomic"
	"time"
	"unsafe"

	"github.com/VictoriaMetrics/VictoriaMetrics/lib/bytesutil"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/encoding"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fasttime"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fs"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/memory"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/mergeset"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/uint64set"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/workingsetcache"
	"github.com/VictoriaMetrics/fastcache"
	xxhash "github.com/cespare/xxhash/v2"
)

const (
	// Prefix for MetricName->TSID entries.
	nsPrefixMetricNameToTSID = 0

	// Prefix for Tag->MetricID entries.
	nsPrefixTagToMetricIDs = 1

	// Prefix for MetricID->TSID entries.
	nsPrefixMetricIDToTSID = 2

	// Prefix for MetricID->MetricName entries.
	nsPrefixMetricIDToMetricName = 3

	// Prefix for deleted MetricID entries.
	nsPrefixDeletedMetricID = 4

	// Prefix for Date->MetricID entries.
	nsPrefixDateToMetricID = 5

	// Prefix for (Date,Tag)->MetricID entries.
	nsPrefixDateTagToMetricIDs = 6
)

// indexDB represents an index db.
type indexDB struct {
	// Atomic counters must go at the top of the structure in order to properly align by 8 bytes on 32-bit archs.
	// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/212 .

	refCount uint64

	// The counter for newly created time series. It can be used for determining time series churn rate.
	newTimeseriesCreated uint64

	// The counter for time series which were re-populated from previous indexDB after the rotation.
	timeseriesRepopulated uint64

	// The number of missing MetricID -> TSID entries.
	// High rate for this value means corrupted indexDB.
	missingTSIDsForMetricID uint64

	// The number of calls for date range searches.
	dateRangeSearchCalls uint64

	// The number of hits for date range searches.
	dateRangeSearchHits uint64

	// The number of calls for global search.
	globalSearchCalls uint64

	// missingMetricNamesForMetricID is a counter of missing MetricID -> MetricName entries.
	// High rate may mean corrupted indexDB due to unclean shutdown.
	// The db must be automatically recovered after that.
	missingMetricNamesForMetricID uint64

	mustDrop uint64

	// generation identifies the index generation ID
	// and is used for syncing items from different indexDBs
	generation uint64

	// The unix timestamp in seconds for the indexDB rotation.
	rotationTimestamp uint64

	name string
	tb   *mergeset.Table

	extDB     *indexDB
	extDBLock sync.Mutex

	// Cache for fast TagFilters -> TSIDs lookup.
	tagFiltersCache *workingsetcache.Cache

	// The parent storage.
	s *Storage

	// Cache for (date, tagFilter) -> loopsCount, which is used for reducing
	// the amount of work when matching a set of filters.
	loopsPerDateTagFilterCache *workingsetcache.Cache

	indexSearchPool sync.Pool
}

// openIndexDB opens index db from the given path.
//
// The last segment of the path should contain unique hex value which
// will be then used as indexDB.generation
//
// The rotationTimestamp must be set to the current unix timestamp when ipenIndexDB
// is called when creating new indexdb during indexdb rotation.
func openIndexDB(path string, s *Storage, rotationTimestamp uint64) (*indexDB, error) {
	if s == nil {
		logger.Panicf("BUG: Storage must be nin-nil")
	}

	name := filepath.Base(path)
	gen, err := strconv.ParseUint(name, 16, 64)
	if err != nil {
		return nil, fmt.Errorf("failed to parse indexdb path %q: %w", path, err)
	}

	tb, err := mergeset.OpenTable(path, invalidateTagFiltersCache, mergeTagToMetricIDsRows)
	if err != nil {
		return nil, fmt.Errorf("cannot open indexDB %q: %w", path, err)
	}

	// Do not persist tagFiltersCache in files, since it is very volatile.
	mem := memory.Allowed()

	db := &indexDB{
		refCount:          1,
		generation:        gen,
		rotationTimestamp: rotationTimestamp,
		tb:                tb,
		name:              name,

		tagFiltersCache:            workingsetcache.New(mem / 32),
		s:                          s,
		loopsPerDateTagFilterCache: workingsetcache.New(mem / 128),
	}
	return db, nil
}

const noDeadline = 1<<64 - 1

// IndexDBMetrics contains essential metrics for indexDB.
type IndexDBMetrics struct {
	TagFiltersCacheSize         uint64
	TagFiltersCacheSizeBytes    uint64
	TagFiltersCacheSizeMaxBytes uint64
	TagFiltersCacheRequests     uint64
	TagFiltersCacheMisses       uint64

	DeletedMetricsCount uint64

	IndexDBRefCount uint64

	NewTimeseriesCreated    uint64
	TimeseriesRepopulated   uint64
	MissingTSIDsForMetricID uint64

	RecentHourMetricIDsSearchCalls uint64
	RecentHourMetricIDsSearchHits  uint64

	DateRangeSearchCalls uint64
	DateRangeSearchHits  uint64
	GlobalSearchCalls    uint64

	MissingMetricNamesForMetricID uint64

	IndexBlocksWithMetricIDsProcessed      uint64
	IndexBlocksWithMetricIDsIncorrectOrder uint64

	MinTimestampForCompositeIndex     uint64
	CompositeFilterSuccessConversions uint64
	CompositeFilterMissingConversions uint64

	mergeset.TableMetrics
}

func (db *indexDB) scheduleToDrop() {
	atomic.AddUint64(&db.mustDrop, 1)
}

// UpdateMetrics updates m with metrics from the db.
func (db *indexDB) UpdateMetrics(m *IndexDBMetrics) {
	var cs fastcache.Stats

	cs.Reset()
	db.tagFiltersCache.UpdateStats(&cs)
	m.TagFiltersCacheSize += cs.EntriesCount
	m.TagFiltersCacheSizeBytes += cs.BytesSize
	m.TagFiltersCacheSizeMaxBytes += cs.MaxBytesSize
	m.TagFiltersCacheRequests += cs.GetCalls
	m.TagFiltersCacheMisses += cs.Misses

	m.DeletedMetricsCount += uint64(db.s.getDeletedMetricIDs().Len())

	m.IndexDBRefCount += atomic.LoadUint64(&db.refCount)
	m.NewTimeseriesCreated += atomic.LoadUint64(&db.newTimeseriesCreated)
	m.TimeseriesRepopulated += atomic.LoadUint64(&db.timeseriesRepopulated)
	m.MissingTSIDsForMetricID += atomic.LoadUint64(&db.missingTSIDsForMetricID)

	m.DateRangeSearchCalls += atomic.LoadUint64(&db.dateRangeSearchCalls)
	m.DateRangeSearchHits += atomic.LoadUint64(&db.dateRangeSearchHits)
	m.GlobalSearchCalls += atomic.LoadUint64(&db.globalSearchCalls)

	m.MissingMetricNamesForMetricID += atomic.LoadUint64(&db.missingMetricNamesForMetricID)

	m.IndexBlocksWithMetricIDsProcessed = atomic.LoadUint64(&indexBlocksWithMetricIDsProcessed)
	m.IndexBlocksWithMetricIDsIncorrectOrder = atomic.LoadUint64(&indexBlocksWithMetricIDsIncorrectOrder)

	m.MinTimestampForCompositeIndex = uint64(db.s.minTimestampForCompositeIndex)
	m.CompositeFilterSuccessConversions = atomic.LoadUint64(&compositeFilterSuccessConversions)
	m.CompositeFilterMissingConversions = atomic.LoadUint64(&compositeFilterMissingConversions)

	db.tb.UpdateMetrics(&m.TableMetrics)
	db.doExtDB(func(extDB *indexDB) {
		extDB.tb.UpdateMetrics(&m.TableMetrics)
		m.IndexDBRefCount += atomic.LoadUint64(&extDB.refCount)
	})
}

func (db *indexDB) doExtDB(f func(extDB *indexDB)) bool {
	db.extDBLock.Lock()
	extDB := db.extDB
	if extDB != nil {
		extDB.incRef()
	}
	db.extDBLock.Unlock()
	if extDB == nil {
		return false
	}
	f(extDB)
	extDB.decRef()
	return true
}

// SetExtDB sets external db to search.
//
// It decrements refCount for the previous extDB.
func (db *indexDB) SetExtDB(extDB *indexDB) {
	db.extDBLock.Lock()
	prevExtDB := db.extDB
	db.extDB = extDB
	db.extDBLock.Unlock()

	if prevExtDB != nil {
		prevExtDB.decRef()
	}
}

// MustClose closes db.
func (db *indexDB) MustClose() {
	db.decRef()
}

func (db *indexDB) incRef() {
	atomic.AddUint64(&db.refCount, 1)
}

func (db *indexDB) decRef() {
	n := atomic.AddUint64(&db.refCount, ^uint64(0))
	if int64(n) < 0 {
		logger.Panicf("BUG: negative refCount: %d", n)
	}
	if n > 0 {
		return
	}

	tbPath := db.tb.Path()
	db.tb.MustClose()
	db.SetExtDB(nil)

	// Free space occupied by caches owned by db.
	db.tagFiltersCache.Stop()
	db.loopsPerDateTagFilterCache.Stop()

	db.tagFiltersCache = nil
	db.s = nil
	db.loopsPerDateTagFilterCache = nil

	if atomic.LoadUint64(&db.mustDrop) == 0 {
		return
	}

	logger.Infof("dropping indexDB %q", tbPath)
	fs.MustRemoveAll(tbPath)
	logger.Infof("indexDB %q has been dropped", tbPath)
}

func (db *indexDB) getFromTagFiltersCache(key []byte) ([]TSID, bool) {
	compressedBuf := tagBufPool.Get()
	defer tagBufPool.Put(compressedBuf)
	compressedBuf.B = db.tagFiltersCache.GetBig(compressedBuf.B[:0], key)
	if len(compressedBuf.B) == 0 {
		return nil, false
	}
	buf := tagBufPool.Get()
	defer tagBufPool.Put(buf)
	var err error
	buf.B, err = encoding.DecompressZSTD(buf.B[:0], compressedBuf.B)
	if err != nil {
		logger.Panicf("FATAL: cannot decompress tsids from tagFiltersCache: %s", err)
	}
	tsids, err := unmarshalTSIDs(nil, buf.B)
	if err != nil {
		logger.Panicf("FATAL: cannot unmarshal tsids from tagFiltersCache: %s", err)
	}
	return tsids, true
}

var tagBufPool bytesutil.ByteBufferPool

func (db *indexDB) putToTagFiltersCache(tsids []TSID, key []byte) {
	buf := tagBufPool.Get()
	buf.B = marshalTSIDs(buf.B[:0], tsids)
	compressedBuf := tagBufPool.Get()
	compressedBuf.B = encoding.CompressZSTDLevel(compressedBuf.B[:0], buf.B, 1)
	tagBufPool.Put(buf)
	db.tagFiltersCache.SetBig(key, compressedBuf.B)
	tagBufPool.Put(compressedBuf)
}

func (db *indexDB) getFromMetricIDCache(dst *TSID, metricID uint64) error {
	// There is no need in prefixing the key with (accountID, projectID),
	// since metricID is globally unique across all (accountID, projectID) values.
	// See getUniqueUint64.

	// There is no need in checking for deleted metricIDs here, since they
	// must be checked by the caller.
	buf := (*[unsafe.Sizeof(*dst)]byte)(unsafe.Pointer(dst))
	key := (*[unsafe.Sizeof(metricID)]byte)(unsafe.Pointer(&metricID))
	tmp := db.s.metricIDCache.Get(buf[:0], key[:])
	if len(tmp) == 0 {
		// The TSID for the given metricID wasn't found in the cache.
		return io.EOF
	}
	if &tmp[0] != &buf[0] || len(tmp) != len(buf) {
		return fmt.Errorf("corrupted MetricID->TSID cache: unexpected size for metricID=%d value; got %d bytes; want %d bytes", metricID, len(tmp), len(buf))
	}
	return nil
}

func (db *indexDB) putToMetricIDCache(metricID uint64, tsid *TSID) {
	buf := (*[unsafe.Sizeof(*tsid)]byte)(unsafe.Pointer(tsid))
	key := (*[unsafe.Sizeof(metricID)]byte)(unsafe.Pointer(&metricID))
	db.s.metricIDCache.Set(key[:], buf[:])
}

func (db *indexDB) getMetricNameFromCache(dst []byte, metricID uint64) []byte {
	// There is no need in prefixing the key with (accountID, projectID),
	// since metricID is globally unique across all (accountID, projectID) values.
	// See getUniqueUint64.

	// There is no need in checking for deleted metricIDs here, since they
	// must be checked by the caller.
	key := (*[unsafe.Sizeof(metricID)]byte)(unsafe.Pointer(&metricID))
	return db.s.metricNameCache.Get(dst, key[:])
}

func (db *indexDB) putMetricNameToCache(metricID uint64, metricName []byte) {
	key := (*[unsafe.Sizeof(metricID)]byte)(unsafe.Pointer(&metricID))
	db.s.metricNameCache.Set(key[:], metricName)
}

// maybeCreateIndexes probabilistically creates indexes for the given (tsid, metricNameRaw) at db.
//
// The probability increases from 0 to 100% during the first hour since db rotation.
//
// It returns true if new index entry was created, and false if it was skipped.
func (db *indexDB) maybeCreateIndexes(tsid *TSID, metricNameRaw []byte) (bool, error) {
	pMin := float64(fasttime.UnixTimestamp()-db.rotationTimestamp) / 3600
	if pMin < 1 {
		p := float64(uint32(fastHashUint64(tsid.MetricID))) / (1 << 32)
		if p > pMin {
			// Fast path: there is no need creating indexes for metricNameRaw yet.
			return false, nil
		}
	}
	// Slow path: create indexes for (tsid, metricNameRaw) at db.
	mn := GetMetricName()
	if err := mn.UnmarshalRaw(metricNameRaw); err != nil {
		return false, fmt.Errorf("cannot unmarshal metricNameRaw %q: %w", metricNameRaw, err)
	}
	mn.sortTags()
	if err := db.createIndexes(tsid, mn); err != nil {
		return false, err
	}
	PutMetricName(mn)
	atomic.AddUint64(&db.timeseriesRepopulated, 1)
	return true, nil
}

func marshalTagFiltersKey(dst []byte, tfss []*TagFilters, tr TimeRange, versioned bool) []byte {
	prefix := ^uint64(0)
	if versioned {
		prefix = atomic.LoadUint64(&tagFiltersKeyGen)
	}
	// Round start and end times to per-day granularity according to per-day inverted index.
	startDate := uint64(tr.MinTimestamp) / msecPerDay
	endDate := uint64(tr.MaxTimestamp) / msecPerDay
	dst = encoding.MarshalUint64(dst, prefix)
	dst = encoding.MarshalUint64(dst, startDate)
	dst = encoding.MarshalUint64(dst, endDate)
	if len(tfss) == 0 {
		return dst
	}
	dst = encoding.MarshalUint32(dst, tfss[0].accountID)
	dst = encoding.MarshalUint32(dst, tfss[0].projectID)
	for _, tfs := range tfss {
		dst = append(dst, 0) // separator between tfs groups.
		for i := range tfs.tfs {
			dst = tfs.tfs[i].MarshalNoAccountIDProjectID(dst)
		}
	}
	return dst
}

func invalidateTagFiltersCache() {
	// This function must be fast, since it is called each
	// time new timeseries is added.
	atomic.AddUint64(&tagFiltersKeyGen, 1)
}

var tagFiltersKeyGen uint64

func marshalTSIDs(dst []byte, tsids []TSID) []byte {
	dst = encoding.MarshalUint64(dst, uint64(len(tsids)))
	for i := range tsids {
		dst = tsids[i].Marshal(dst)
	}
	return dst
}

func unmarshalTSIDs(dst []TSID, src []byte) ([]TSID, error) {
	if len(src) < 8 {
		return dst, fmt.Errorf("cannot unmarshal the number of tsids from %d bytes; require at least %d bytes", len(src), 8)
	}
	n := encoding.UnmarshalUint64(src)
	src = src[8:]
	dstLen := len(dst)
	if nn := dstLen + int(n) - cap(dst); nn > 0 {
		dst = append(dst[:cap(dst)], make([]TSID, nn)...)
	}
	dst = dst[:dstLen+int(n)]
	for i := 0; i < int(n); i++ {
		tail, err := dst[dstLen+i].Unmarshal(src)
		if err != nil {
			return dst, fmt.Errorf("cannot unmarshal tsid #%d out of %d: %w", i, n, err)
		}
		src = tail
	}
	if len(src) > 0 {
		return dst, fmt.Errorf("non-zero tail left after unmarshaling %d tsids; len(tail)=%d", n, len(src))
	}
	return dst, nil
}

// getTSIDByNameNoCreate fills the dst with TSID for the given metricName.
//
// It returns io.EOF if the given mn isn't found locally.
func (db *indexDB) getTSIDByNameNoCreate(dst *TSID, metricName []byte) error {
	is := db.getIndexSearch(0, 0, noDeadline)
	err := is.getTSIDByMetricName(dst, metricName)
	db.putIndexSearch(is)
	if err == nil {
		return nil
	}
	if err != io.EOF {
		return fmt.Errorf("cannot search TSID by MetricName %q: %w", metricName, err)
	}

	// Do not search for the TSID in the external storage,
	// since this function is already called by another indexDB instance.

	// The TSID for the given mn wasn't found.
	return io.EOF
}

type indexSearch struct {
	db *indexDB
	ts mergeset.TableSearch
	kb bytesutil.ByteBuffer
	mp tagToMetricIDsRowParser

	accountID uint32
	projectID uint32

	// deadline in unix timestamp seconds for the given search.
	deadline uint64

	// tsidByNameMisses and tsidByNameSkips is used for a performance
	// hack in GetOrCreateTSIDByName. See the comment there.
	tsidByNameMisses int
	tsidByNameSkips  int
}

// GetOrCreateTSIDByName fills the dst with TSID for the given metricName.
func (is *indexSearch) GetOrCreateTSIDByName(dst *TSID, metricName []byte) error {
	// A hack: skip searching for the TSID after many serial misses.
	// This should improve insertion performance for big batches
	// of new time series.
	if is.tsidByNameMisses < 100 {
		err := is.getTSIDByMetricName(dst, metricName)
		if err == nil {
			is.tsidByNameMisses = 0
			return nil
		}
		if err != io.EOF {
			return fmt.Errorf("cannot search TSID by MetricName %q: %w", metricName, err)
		}
		is.tsidByNameMisses++
	} else {
		is.tsidByNameSkips++
		if is.tsidByNameSkips > 10000 {
			is.tsidByNameSkips = 0
			is.tsidByNameMisses = 0
		}
	}

	// TSID for the given name wasn't found. Create it.
	// It is OK if duplicate TSID for mn is created by concurrent goroutines.
	// Metric results will be merged by mn after TableSearch.
	if err := is.db.createTSIDByName(dst, metricName); err != nil {
		return fmt.Errorf("cannot create TSID by MetricName %q: %w", metricName, err)
	}
	return nil
}

func (db *indexDB) getIndexSearch(accountID, projectID uint32, deadline uint64) *indexSearch {
	v := db.indexSearchPool.Get()
	if v == nil {
		v = &indexSearch{
			db: db,
		}
	}
	is := v.(*indexSearch)
	is.ts.Init(db.tb)
	is.accountID = accountID
	is.projectID = projectID
	is.deadline = deadline
	return is
}

func (db *indexDB) putIndexSearch(is *indexSearch) {
	is.ts.MustClose()
	is.kb.Reset()
	is.mp.Reset()
	is.accountID = 0
	is.projectID = 0
	is.deadline = 0

	// Do not reset tsidByNameMisses and tsidByNameSkips,
	// since they are used in GetOrCreateTSIDByName across call boundaries.

	db.indexSearchPool.Put(is)
}

func (db *indexDB) createTSIDByName(dst *TSID, metricName []byte) error {
	mn := GetMetricName()
	defer PutMetricName(mn)
	if err := mn.Unmarshal(metricName); err != nil {
		return fmt.Errorf("cannot unmarshal metricName %q: %w", metricName, err)
	}

	created, err := db.getOrCreateTSID(dst, metricName, mn)
	if err != nil {
		return fmt.Errorf("cannot generate TSID: %w", err)
	}
	if err := db.createIndexes(dst, mn); err != nil {
		return fmt.Errorf("cannot create indexes: %w", err)
	}

	// There is no need in invalidating tag cache, since it is invalidated
	// on db.tb flush via invalidateTagFiltersCache flushCallback passed to OpenTable.

	if created {
		// Increase the newTimeseriesCreated counter only if tsid wasn't found in indexDB
		atomic.AddUint64(&db.newTimeseriesCreated, 1)
		if logNewSeries {
			logger.Infof("new series created: %s", mn.String())
		}
	}
	return nil
}

// SetLogNewSeries updates new series logging.
//
// This function must be called before any calling any storage functions.
func SetLogNewSeries(ok bool) {
	logNewSeries = ok
}

var logNewSeries = false

// getOrCreateTSID looks for existing TSID for the given metricName in db.extDB or creates a new TSID if nothing was found.
//
// Returns true if TSID was created or false if TSID was in extDB
func (db *indexDB) getOrCreateTSID(dst *TSID, metricName []byte, mn *MetricName) (bool, error) {
	// Search the TSID in the external storage.
	// This is usually the db from the previous period.
	var err error
	if db.doExtDB(func(extDB *indexDB) {
		err = extDB.getTSIDByNameNoCreate(dst, metricName)
	}) {
		if err == nil {
			// The TSID has been found in the external storage.
			return false, nil
		}
		if err != io.EOF {
			return false, fmt.Errorf("external search failed: %w", err)
		}
	}
	// The TSID wasn't found in the external storage.
	// Generate it locally.
	generateTSID(dst, mn)
	return true, nil
}

func generateTSID(dst *TSID, mn *MetricName) {
	dst.AccountID = mn.AccountID
	dst.ProjectID = mn.ProjectID
	dst.MetricGroupID = xxhash.Sum64(mn.MetricGroup)
	// Assume that the job-like metric is put at mn.Tags[0], while instance-like metric is put at mn.Tags[1]
	// This assumption is true because mn.Tags must be sorted with mn.sortTags() before calling generateTSID() function.
	// This allows grouping data blocks for the same (job, instance) close to each other on disk.
	// This reduces disk seeks and disk read IO when data blocks are read from disk for the same job and/or instance.
	// For example, data blocks for time series matching `process_resident_memory_bytes{job="vmstorage"}` are physically adjancent on disk.
	if len(mn.Tags) > 0 {
		dst.JobID = uint32(xxhash.Sum64(mn.Tags[0].Value))
	}
	if len(mn.Tags) > 1 {
		dst.InstanceID = uint32(xxhash.Sum64(mn.Tags[1].Value))
	}
	dst.MetricID = generateUniqueMetricID()
}

func (db *indexDB) createIndexes(tsid *TSID, mn *MetricName) error {
	// The order of index items is important.
	// It guarantees index consistency.

	ii := getIndexItems()
	defer putIndexItems(ii)

	// Create MetricName -> TSID index.
	ii.B = append(ii.B, nsPrefixMetricNameToTSID)
	ii.B = mn.Marshal(ii.B)
	ii.B = append(ii.B, kvSeparatorChar)
	ii.B = tsid.Marshal(ii.B)
	ii.Next()

	// Create MetricID -> MetricName index.
	ii.B = marshalCommonPrefix(ii.B, nsPrefixMetricIDToMetricName, mn.AccountID, mn.ProjectID)
	ii.B = encoding.MarshalUint64(ii.B, tsid.MetricID)
	ii.B = mn.Marshal(ii.B)
	ii.Next()

	// Create MetricID -> TSID index.
	ii.B = marshalCommonPrefix(ii.B, nsPrefixMetricIDToTSID, mn.AccountID, mn.ProjectID)
	ii.B = encoding.MarshalUint64(ii.B, tsid.MetricID)
	ii.B = tsid.Marshal(ii.B)
	ii.Next()

	prefix := kbPool.Get()
	prefix.B = marshalCommonPrefix(prefix.B[:0], nsPrefixTagToMetricIDs, mn.AccountID, mn.ProjectID)
	ii.registerTagIndexes(prefix.B, mn, tsid.MetricID)
	kbPool.Put(prefix)

	return db.tb.AddItems(ii.Items)
}

type indexItems struct {
	B     []byte
	Items [][]byte

	start int
}

func (ii *indexItems) reset() {
	ii.B = ii.B[:0]
	ii.Items = ii.Items[:0]
	ii.start = 0
}

func (ii *indexItems) Next() {
	ii.Items = append(ii.Items, ii.B[ii.start:])
	ii.start = len(ii.B)
}

func getIndexItems() *indexItems {
	v := indexItemsPool.Get()
	if v == nil {
		return &indexItems{}
	}
	return v.(*indexItems)
}

func putIndexItems(ii *indexItems) {
	ii.reset()
	indexItemsPool.Put(ii)
}

var indexItemsPool sync.Pool

// SearchTagKeysOnTimeRange returns all the tag keys on the given tr.
func (db *indexDB) SearchTagKeysOnTimeRange(accountID, projectID uint32, tr TimeRange, maxTagKeys int, deadline uint64) ([]string, error) {
	tks := make(map[string]struct{})
	is := db.getIndexSearch(accountID, projectID, deadline)
	err := is.searchTagKeysOnTimeRange(tks, tr, maxTagKeys)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}

	ok := db.doExtDB(func(extDB *indexDB) {
		is := extDB.getIndexSearch(accountID, projectID, deadline)
		err = is.searchTagKeysOnTimeRange(tks, tr, maxTagKeys)
		extDB.putIndexSearch(is)
	})
	if ok && err != nil {
		return nil, err
	}

	keys := make([]string, 0, len(tks))
	for key := range tks {
		// Do not skip empty keys, since they are converted to __name__
		keys = append(keys, key)
	}
	// Do not sort keys, since they must be sorted by vmselect.
	return keys, nil
}

func (is *indexSearch) searchTagKeysOnTimeRange(tks map[string]struct{}, tr TimeRange, maxTagKeys int) error {
	minDate := uint64(tr.MinTimestamp) / msecPerDay
	maxDate := uint64(tr.MaxTimestamp) / msecPerDay
	if minDate > maxDate || maxDate-minDate > maxDaysForPerDaySearch {
		return is.searchTagKeys(tks, maxTagKeys)
	}
	var mu sync.Mutex
	wg := getWaitGroup()
	var errGlobal error
	for date := minDate; date <= maxDate; date++ {
		wg.Add(1)
		go func(date uint64) {
			defer wg.Done()
			tksLocal := make(map[string]struct{})
			isLocal := is.db.getIndexSearch(is.accountID, is.projectID, is.deadline)
			err := isLocal.searchTagKeysOnDate(tksLocal, date, maxTagKeys)
			is.db.putIndexSearch(isLocal)
			mu.Lock()
			defer mu.Unlock()
			if errGlobal != nil {
				return
			}
			if err != nil {
				errGlobal = err
				return
			}
			if len(tks) >= maxTagKeys {
				return
			}
			for k := range tksLocal {
				tks[k] = struct{}{}
			}
		}(date)
	}
	wg.Wait()
	putWaitGroup(wg)
	return errGlobal
}

func (is *indexSearch) searchTagKeysOnDate(tks map[string]struct{}, date uint64, maxTagKeys int) error {
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	mp.Reset()
	dmis := is.db.s.getDeletedMetricIDs()
	loopsPaceLimiter := 0
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixDateTagToMetricIDs)
	kb.B = encoding.MarshalUint64(kb.B, date)
	prefix := kb.B
	ts.Seek(prefix)
	for len(tks) < maxTagKeys && ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			break
		}
		if err := mp.Init(item, nsPrefixDateTagToMetricIDs); err != nil {
			return err
		}
		if mp.IsDeletedTag(dmis) {
			continue
		}
		key := mp.Tag.Key
		if !isArtificialTagKey(key) {
			tks[string(key)] = struct{}{}
		}

		// Search for the next tag key.
		// The last char in kb.B must be tagSeparatorChar.
		// Just increment it in order to jump to the next tag key.
		kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixDateTagToMetricIDs)
		kb.B = encoding.MarshalUint64(kb.B, date)
		if len(key) > 0 && key[0] == compositeTagKeyPrefix {
			// skip composite tag entries
			kb.B = append(kb.B, compositeTagKeyPrefix)
		} else {
			kb.B = marshalTagValue(kb.B, key)
		}
		kb.B[len(kb.B)-1]++
		ts.Seek(kb.B)
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error during search for prefix %q: %w", prefix, err)
	}
	return nil
}

// SearchTagKeys returns all the tag keys for the given accountID, projectID.
func (db *indexDB) SearchTagKeys(accountID, projectID uint32, maxTagKeys int, deadline uint64) ([]string, error) {
	tks := make(map[string]struct{})

	is := db.getIndexSearch(accountID, projectID, deadline)
	err := is.searchTagKeys(tks, maxTagKeys)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}

	ok := db.doExtDB(func(extDB *indexDB) {
		is := extDB.getIndexSearch(accountID, projectID, deadline)
		err = is.searchTagKeys(tks, maxTagKeys)
		extDB.putIndexSearch(is)
	})
	if ok && err != nil {
		return nil, err
	}

	keys := make([]string, 0, len(tks))
	for key := range tks {
		// Do not skip empty keys, since they are converted to __name__
		keys = append(keys, key)
	}
	// Do not sort keys, since they must be sorted by vmselect.
	return keys, nil
}

func (is *indexSearch) searchTagKeys(tks map[string]struct{}, maxTagKeys int) error {
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	mp.Reset()
	dmis := is.db.s.getDeletedMetricIDs()
	loopsPaceLimiter := 0
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	prefix := kb.B
	ts.Seek(prefix)
	for len(tks) < maxTagKeys && ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			break
		}
		if err := mp.Init(item, nsPrefixTagToMetricIDs); err != nil {
			return err
		}
		if mp.IsDeletedTag(dmis) {
			continue
		}
		key := mp.Tag.Key
		if !isArtificialTagKey(key) {
			tks[string(key)] = struct{}{}
		}

		// Search for the next tag key.
		// The last char in kb.B must be tagSeparatorChar.
		// Just increment it in order to jump to the next tag key.
		kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
		if len(key) > 0 && key[0] == compositeTagKeyPrefix {
			// skip composite tag entries
			kb.B = append(kb.B, compositeTagKeyPrefix)
		} else {
			kb.B = marshalTagValue(kb.B, key)
		}
		kb.B[len(kb.B)-1]++
		ts.Seek(kb.B)
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error during search for prefix %q: %w", prefix, err)
	}
	return nil
}

// SearchTagValuesOnTimeRange returns all the tag values for the given tagKey on tr.
func (db *indexDB) SearchTagValuesOnTimeRange(accountID, projectID uint32, tagKey []byte, tr TimeRange, maxTagValues int, deadline uint64) ([]string, error) {
	tvs := make(map[string]struct{})
	is := db.getIndexSearch(accountID, projectID, deadline)
	err := is.searchTagValuesOnTimeRange(tvs, tagKey, tr, maxTagValues)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}
	ok := db.doExtDB(func(extDB *indexDB) {
		is := extDB.getIndexSearch(accountID, projectID, deadline)
		err = is.searchTagValuesOnTimeRange(tvs, tagKey, tr, maxTagValues)
		extDB.putIndexSearch(is)
	})
	if ok && err != nil {
		return nil, err
	}

	tagValues := make([]string, 0, len(tvs))
	for tv := range tvs {
		if len(tv) == 0 {
			// Skip empty values, since they have no any meaning.
			// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/600
			continue
		}
		tagValues = append(tagValues, tv)
	}
	// Do not sort tagValues, since they must be sorted by vmselect.
	return tagValues, nil
}

func (is *indexSearch) searchTagValuesOnTimeRange(tvs map[string]struct{}, tagKey []byte, tr TimeRange, maxTagValues int) error {
	minDate := uint64(tr.MinTimestamp) / msecPerDay
	maxDate := uint64(tr.MaxTimestamp) / msecPerDay
	if minDate > maxDate || maxDate-minDate > maxDaysForPerDaySearch {
		return is.searchTagValues(tvs, tagKey, maxTagValues)
	}
	var mu sync.Mutex
	wg := getWaitGroup()
	var errGlobal error
	for date := minDate; date <= maxDate; date++ {
		wg.Add(1)
		go func(date uint64) {
			defer wg.Done()
			tvsLocal := make(map[string]struct{})
			isLocal := is.db.getIndexSearch(is.accountID, is.projectID, is.deadline)
			err := isLocal.searchTagValuesOnDate(tvsLocal, tagKey, date, maxTagValues)
			is.db.putIndexSearch(isLocal)
			mu.Lock()
			defer mu.Unlock()
			if errGlobal != nil {
				return
			}
			if err != nil {
				errGlobal = err
				return
			}
			if len(tvs) >= maxTagValues {
				return
			}
			for v := range tvsLocal {
				tvs[v] = struct{}{}
			}
		}(date)
	}
	wg.Wait()
	putWaitGroup(wg)
	return errGlobal
}

func (is *indexSearch) searchTagValuesOnDate(tvs map[string]struct{}, tagKey []byte, date uint64, maxTagValues int) error {
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	mp.Reset()
	dmis := is.db.s.getDeletedMetricIDs()
	loopsPaceLimiter := 0
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixDateTagToMetricIDs)
	kb.B = encoding.MarshalUint64(kb.B, date)
	kb.B = marshalTagValue(kb.B, tagKey)
	prefix := kb.B
	ts.Seek(prefix)
	for len(tvs) < maxTagValues && ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			break
		}
		if err := mp.Init(item, nsPrefixDateTagToMetricIDs); err != nil {
			return err
		}
		if mp.IsDeletedTag(dmis) {
			continue
		}
		if string(mp.Tag.Key) != string(tagKey) {
			break
		}
		tvs[string(mp.Tag.Value)] = struct{}{}
		if mp.MetricIDsLen() < maxMetricIDsPerRow/2 {
			// There is no need in searching for the next tag value,
			// since it is likely it is located in the next row,
			// because the current row contains incomplete metricIDs set.
			continue
		}
		// Search for the next tag value.
		// The last char in kb.B must be tagSeparatorChar.
		// Just increment it in order to jump to the next tag value.
		kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixDateTagToMetricIDs)
		kb.B = encoding.MarshalUint64(kb.B, date)
		kb.B = marshalTagValue(kb.B, mp.Tag.Key)
		kb.B = marshalTagValue(kb.B, mp.Tag.Value)
		kb.B[len(kb.B)-1]++
		ts.Seek(kb.B)
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for tag name prefix %q: %w", prefix, err)
	}
	return nil
}

// SearchTagValues returns all the tag values for the given tagKey
func (db *indexDB) SearchTagValues(accountID, projectID uint32, tagKey []byte, maxTagValues int, deadline uint64) ([]string, error) {
	tvs := make(map[string]struct{})
	is := db.getIndexSearch(accountID, projectID, deadline)
	err := is.searchTagValues(tvs, tagKey, maxTagValues)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}
	ok := db.doExtDB(func(extDB *indexDB) {
		is := extDB.getIndexSearch(accountID, projectID, deadline)
		err = is.searchTagValues(tvs, tagKey, maxTagValues)
		extDB.putIndexSearch(is)
	})
	if ok && err != nil {
		return nil, err
	}

	tagValues := make([]string, 0, len(tvs))
	for tv := range tvs {
		if len(tv) == 0 {
			// Skip empty values, since they have no any meaning.
			// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/600
			continue
		}
		tagValues = append(tagValues, tv)
	}
	// Do not sort tagValues, since they must be sorted by vmselect.
	return tagValues, nil
}

func (is *indexSearch) searchTagValues(tvs map[string]struct{}, tagKey []byte, maxTagValues int) error {
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	mp.Reset()
	dmis := is.db.s.getDeletedMetricIDs()
	loopsPaceLimiter := 0
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	kb.B = marshalTagValue(kb.B, tagKey)
	prefix := kb.B
	ts.Seek(prefix)
	for len(tvs) < maxTagValues && ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			break
		}
		if err := mp.Init(item, nsPrefixTagToMetricIDs); err != nil {
			return err
		}
		if mp.IsDeletedTag(dmis) {
			continue
		}
		if string(mp.Tag.Key) != string(tagKey) {
			break
		}
		tvs[string(mp.Tag.Value)] = struct{}{}
		if mp.MetricIDsLen() < maxMetricIDsPerRow/2 {
			// There is no need in searching for the next tag value,
			// since it is likely it is located in the next row,
			// because the current row contains incomplete metricIDs set.
			continue
		}
		// Search for the next tag value.
		// The last char in kb.B must be tagSeparatorChar.
		// Just increment it in order to jump to the next tag value.
		kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
		kb.B = marshalTagValue(kb.B, mp.Tag.Key)
		kb.B = marshalTagValue(kb.B, mp.Tag.Value)
		kb.B[len(kb.B)-1]++
		ts.Seek(kb.B)
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for tag name prefix %q: %w", prefix, err)
	}
	return nil
}

// SearchTagValueSuffixes returns all the tag value suffixes for the given tagKey and tagValuePrefix on the given tr.
//
// This allows implementing https://graphite-api.readthedocs.io/en/latest/api.html#metrics-find or similar APIs.
//
// If it returns maxTagValueSuffixes suffixes, then it is likely more than maxTagValueSuffixes suffixes is found.
func (db *indexDB) SearchTagValueSuffixes(accountID, projectID uint32, tr TimeRange, tagKey, tagValuePrefix []byte,
	delimiter byte, maxTagValueSuffixes int, deadline uint64) ([]string, error) {
	// TODO: cache results?

	tvss := make(map[string]struct{})
	is := db.getIndexSearch(accountID, projectID, deadline)
	err := is.searchTagValueSuffixesForTimeRange(tvss, tr, tagKey, tagValuePrefix, delimiter, maxTagValueSuffixes)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}
	if len(tvss) < maxTagValueSuffixes {
		ok := db.doExtDB(func(extDB *indexDB) {
			is := extDB.getIndexSearch(accountID, projectID, deadline)
			err = is.searchTagValueSuffixesForTimeRange(tvss, tr, tagKey, tagValuePrefix, delimiter, maxTagValueSuffixes)
			extDB.putIndexSearch(is)
		})
		if ok && err != nil {
			return nil, err
		}
	}

	suffixes := make([]string, 0, len(tvss))
	for suffix := range tvss {
		// Do not skip empty suffixes, since they may represent leaf tag values.
		suffixes = append(suffixes, suffix)
	}
	if len(suffixes) > maxTagValueSuffixes {
		suffixes = suffixes[:maxTagValueSuffixes]
	}
	// Do not sort suffixes, since they must be sorted by vmselect.
	return suffixes, nil
}

func (is *indexSearch) searchTagValueSuffixesForTimeRange(tvss map[string]struct{}, tr TimeRange, tagKey, tagValuePrefix []byte, delimiter byte, maxTagValueSuffixes int) error {
	minDate := uint64(tr.MinTimestamp) / msecPerDay
	maxDate := uint64(tr.MaxTimestamp) / msecPerDay
	if minDate > maxDate || maxDate-minDate > maxDaysForPerDaySearch {
		return is.searchTagValueSuffixesAll(tvss, tagKey, tagValuePrefix, delimiter, maxTagValueSuffixes)
	}
	// Query over multiple days in parallel.
	wg := getWaitGroup()
	var errGlobal error
	var mu sync.Mutex // protects tvss + errGlobal from concurrent access below.
	for minDate <= maxDate {
		wg.Add(1)
		go func(date uint64) {
			defer wg.Done()
			tvssLocal := make(map[string]struct{})
			isLocal := is.db.getIndexSearch(is.accountID, is.projectID, is.deadline)
			err := isLocal.searchTagValueSuffixesForDate(tvssLocal, date, tagKey, tagValuePrefix, delimiter, maxTagValueSuffixes)
			is.db.putIndexSearch(isLocal)
			mu.Lock()
			defer mu.Unlock()
			if errGlobal != nil {
				return
			}
			if err != nil {
				errGlobal = err
				return
			}
			if len(tvss) > maxTagValueSuffixes {
				return
			}
			for k := range tvssLocal {
				tvss[k] = struct{}{}
			}
		}(minDate)
		minDate++
	}
	wg.Wait()
	putWaitGroup(wg)
	return errGlobal
}

func (is *indexSearch) searchTagValueSuffixesAll(tvss map[string]struct{}, tagKey, tagValuePrefix []byte, delimiter byte, maxTagValueSuffixes int) error {
	kb := &is.kb
	nsPrefix := byte(nsPrefixTagToMetricIDs)
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefix)
	kb.B = marshalTagValue(kb.B, tagKey)
	kb.B = marshalTagValue(kb.B, tagValuePrefix)
	kb.B = kb.B[:len(kb.B)-1] // remove tagSeparatorChar from the end of kb.B
	prefix := append([]byte(nil), kb.B...)
	return is.searchTagValueSuffixesForPrefix(tvss, nsPrefix, prefix, len(tagValuePrefix), delimiter, maxTagValueSuffixes)
}

func (is *indexSearch) searchTagValueSuffixesForDate(tvss map[string]struct{}, date uint64, tagKey, tagValuePrefix []byte, delimiter byte, maxTagValueSuffixes int) error {
	nsPrefix := byte(nsPrefixDateTagToMetricIDs)
	kb := &is.kb
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefix)
	kb.B = encoding.MarshalUint64(kb.B, date)
	kb.B = marshalTagValue(kb.B, tagKey)
	kb.B = marshalTagValue(kb.B, tagValuePrefix)
	kb.B = kb.B[:len(kb.B)-1] // remove tagSeparatorChar from the end of kb.B
	prefix := append([]byte(nil), kb.B...)
	return is.searchTagValueSuffixesForPrefix(tvss, nsPrefix, prefix, len(tagValuePrefix), delimiter, maxTagValueSuffixes)
}

func (is *indexSearch) searchTagValueSuffixesForPrefix(tvss map[string]struct{}, nsPrefix byte, prefix []byte, tagValuePrefixLen int, delimiter byte, maxTagValueSuffixes int) error {
	kb := &is.kb
	ts := &is.ts
	mp := &is.mp
	mp.Reset()
	dmis := is.db.s.getDeletedMetricIDs()
	loopsPaceLimiter := 0
	ts.Seek(prefix)
	for len(tvss) < maxTagValueSuffixes && ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			break
		}
		if err := mp.Init(item, nsPrefix); err != nil {
			return err
		}
		if mp.IsDeletedTag(dmis) {
			continue
		}
		tagValue := mp.Tag.Value
		suffix := tagValue[tagValuePrefixLen:]
		n := bytes.IndexByte(suffix, delimiter)
		if n < 0 {
			// Found leaf tag value that doesn't have delimiters after the given tagValuePrefix.
			tvss[string(suffix)] = struct{}{}
			continue
		}
		// Found non-leaf tag value. Extract suffix that end with the given delimiter.
		suffix = suffix[:n+1]
		tvss[string(suffix)] = struct{}{}
		if suffix[len(suffix)-1] == 255 {
			continue
		}
		// Search for the next suffix
		suffix[len(suffix)-1]++
		kb.B = append(kb.B[:0], prefix...)
		kb.B = marshalTagValue(kb.B, suffix)
		kb.B = kb.B[:len(kb.B)-1] // remove tagSeparatorChar
		ts.Seek(kb.B)
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for tag value sufixes for prefix %q: %w", prefix, err)
	}
	return nil
}

// GetSeriesCount returns the approximate number of unique timeseries for the given (accountID, projectID).
//
// It includes the deleted series too and may count the same series
// up to two times - in db and extDB.
func (db *indexDB) GetSeriesCount(accountID, projectID uint32, deadline uint64) (uint64, error) {
	is := db.getIndexSearch(accountID, projectID, deadline)
	n, err := is.getSeriesCount()
	db.putIndexSearch(is)
	if err != nil {
		return 0, err
	}

	var nExt uint64
	ok := db.doExtDB(func(extDB *indexDB) {
		is := extDB.getIndexSearch(accountID, projectID, deadline)
		nExt, err = is.getSeriesCount()
		extDB.putIndexSearch(is)
	})
	if ok && err != nil {
		return 0, fmt.Errorf("error when searching in extDB: %w", err)
	}
	return n + nExt, nil
}

func (is *indexSearch) getSeriesCount() (uint64, error) {
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	loopsPaceLimiter := 0
	var metricIDsLen uint64
	// Extract the number of series from ((__name__=value): metricIDs) rows
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	kb.B = marshalTagValue(kb.B, nil)
	ts.Seek(kb.B)
	for ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return 0, err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, kb.B) {
			break
		}
		tail := item[len(kb.B):]
		n := bytes.IndexByte(tail, tagSeparatorChar)
		if n < 0 {
			return 0, fmt.Errorf("invalid tag->metricIDs line %q: cannot find tagSeparatorChar %d", item, tagSeparatorChar)
		}
		tail = tail[n+1:]
		if err := mp.InitOnlyTail(item, tail); err != nil {
			return 0, err
		}
		// Take into account deleted timeseries too.
		// It is OK if series can be counted multiple times in rare cases -
		// the returned number is an estimation.
		metricIDsLen += uint64(mp.MetricIDsLen())
	}
	if err := ts.Error(); err != nil {
		return 0, fmt.Errorf("error when counting unique timeseries: %w", err)
	}
	return metricIDsLen, nil
}

// GetTSDBStatusWithFiltersForDate returns topN entries for tsdb status for the given tfss, date, accountID and projectID.
func (db *indexDB) GetTSDBStatusWithFiltersForDate(accountID, projectID uint32, tfss []*TagFilters, date uint64, topN, maxMetrics int, deadline uint64) (*TSDBStatus, error) {
	is := db.getIndexSearch(accountID, projectID, deadline)
	status, err := is.getTSDBStatusWithFiltersForDate(tfss, date, topN, maxMetrics)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}
	if status.hasEntries() {
		return status, nil
	}
	ok := db.doExtDB(func(extDB *indexDB) {
		is := extDB.getIndexSearch(accountID, projectID, deadline)
		status, err = is.getTSDBStatusWithFiltersForDate(tfss, date, topN, maxMetrics)
		extDB.putIndexSearch(is)
	})
	if ok && err != nil {
		return nil, fmt.Errorf("error when obtaining TSDB status from extDB: %w", err)
	}
	return status, nil
}

// getTSDBStatusWithFiltersForDate returns topN entries for tsdb status for the given tfss and the given date.
func (is *indexSearch) getTSDBStatusWithFiltersForDate(tfss []*TagFilters, date uint64, topN, maxMetrics int) (*TSDBStatus, error) {
	var filter *uint64set.Set
	if len(tfss) > 0 {
		tr := TimeRange{
			MinTimestamp: int64(date) * msecPerDay,
			MaxTimestamp: int64(date+1) * msecPerDay,
		}
		metricIDs, err := is.searchMetricIDsInternal(tfss, tr, maxMetrics)
		if err != nil {
			return nil, err
		}
		if metricIDs.Len() == 0 {
			// Nothing found.
			return &TSDBStatus{}, nil
		}
		filter = metricIDs
	}

	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	thLabelValueCountByLabelName := newTopHeap(topN)
	thSeriesCountByLabelValuePair := newTopHeap(topN)
	thSeriesCountByMetricName := newTopHeap(topN)
	var tmp, labelName, labelNameValue []byte
	var labelValueCountByLabelName, seriesCountByLabelValuePair uint64
	nameEqualBytes := []byte("__name__=")

	loopsPaceLimiter := 0
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixDateTagToMetricIDs)
	kb.B = encoding.MarshalUint64(kb.B, date)
	prefix := kb.B
	ts.Seek(prefix)
	for ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return nil, err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			break
		}
		matchingSeriesCount := 0
		if filter != nil {
			if err := mp.Init(item, nsPrefixDateTagToMetricIDs); err != nil {
				return nil, err
			}
			mp.ParseMetricIDs()
			for _, metricID := range mp.MetricIDs {
				if filter.Has(metricID) {
					matchingSeriesCount++
				}
			}
			if matchingSeriesCount == 0 {
				// Skip rows without matching metricIDs.
				continue
			}
		}
		tail := item[len(prefix):]
		var err error
		tail, tmp, err = unmarshalTagValue(tmp[:0], tail)
		if err != nil {
			return nil, fmt.Errorf("cannot unmarshal tag key from line %q: %w", item, err)
		}
		if isArtificialTagKey(tmp) {
			// Skip artificially created tag keys.
			kb.B = append(kb.B[:0], prefix...)
			if len(tmp) > 0 && tmp[0] == compositeTagKeyPrefix {
				kb.B = append(kb.B, compositeTagKeyPrefix)
			} else {
				kb.B = marshalTagValue(kb.B, tmp)
			}
			kb.B[len(kb.B)-1]++
			ts.Seek(kb.B)
			continue
		}
		if len(tmp) == 0 {
			tmp = append(tmp, "__name__"...)
		}
		if !bytes.Equal(tmp, labelName) {
			thLabelValueCountByLabelName.pushIfNonEmpty(labelName, labelValueCountByLabelName)
			labelValueCountByLabelName = 0
			labelName = append(labelName[:0], tmp...)
		}
		tmp = append(tmp, '=')
		tail, tmp, err = unmarshalTagValue(tmp, tail)
		if err != nil {
			return nil, fmt.Errorf("cannot unmarshal tag value from line %q: %w", item, err)
		}
		if !bytes.Equal(tmp, labelNameValue) {
			thSeriesCountByLabelValuePair.pushIfNonEmpty(labelNameValue, seriesCountByLabelValuePair)
			if bytes.HasPrefix(labelNameValue, nameEqualBytes) {
				thSeriesCountByMetricName.pushIfNonEmpty(labelNameValue[len(nameEqualBytes):], seriesCountByLabelValuePair)
			}
			seriesCountByLabelValuePair = 0
			labelValueCountByLabelName++
			labelNameValue = append(labelNameValue[:0], tmp...)
		}
		if filter == nil {
			if err := mp.InitOnlyTail(item, tail); err != nil {
				return nil, err
			}
			matchingSeriesCount = mp.MetricIDsLen()
		}
		// Take into account deleted timeseries too.
		// It is OK if series can be counted multiple times in rare cases -
		// the returned number is an estimation.
		seriesCountByLabelValuePair += uint64(matchingSeriesCount)
	}
	if err := ts.Error(); err != nil {
		return nil, fmt.Errorf("error when counting time series by metric names: %w", err)
	}
	thLabelValueCountByLabelName.pushIfNonEmpty(labelName, labelValueCountByLabelName)
	thSeriesCountByLabelValuePair.pushIfNonEmpty(labelNameValue, seriesCountByLabelValuePair)
	if bytes.HasPrefix(labelNameValue, nameEqualBytes) {
		thSeriesCountByMetricName.pushIfNonEmpty(labelNameValue[len(nameEqualBytes):], seriesCountByLabelValuePair)
	}
	status := &TSDBStatus{
		SeriesCountByMetricName:     thSeriesCountByMetricName.getSortedResult(),
		LabelValueCountByLabelName:  thLabelValueCountByLabelName.getSortedResult(),
		SeriesCountByLabelValuePair: thSeriesCountByLabelValuePair.getSortedResult(),
	}
	return status, nil
}

// TSDBStatus contains TSDB status data for /api/v1/status/tsdb.
//
// See https://prometheus.io/docs/prometheus/latest/querying/api/#tsdb-stats
type TSDBStatus struct {
	SeriesCountByMetricName     []TopHeapEntry
	LabelValueCountByLabelName  []TopHeapEntry
	SeriesCountByLabelValuePair []TopHeapEntry
}

func (status *TSDBStatus) hasEntries() bool {
	return len(status.SeriesCountByLabelValuePair) > 0
}

// topHeap maintains a heap of topHeapEntries with the maximum TopHeapEntry.n values.
type topHeap struct {
	topN int
	a    []TopHeapEntry
}

// newTopHeap returns topHeap for topN items.
func newTopHeap(topN int) *topHeap {
	return &topHeap{
		topN: topN,
	}
}

// TopHeapEntry represents an entry from `top heap` used in stats.
type TopHeapEntry struct {
	Name  string
	Count uint64
}

func (th *topHeap) pushIfNonEmpty(name []byte, count uint64) {
	if count == 0 {
		return
	}
	if len(th.a) < th.topN {
		th.a = append(th.a, TopHeapEntry{
			Name:  string(name),
			Count: count,
		})
		heap.Fix(th, len(th.a)-1)
		return
	}
	if count <= th.a[0].Count {
		return
	}
	th.a[0] = TopHeapEntry{
		Name:  string(name),
		Count: count,
	}
	heap.Fix(th, 0)
}

func (th *topHeap) getSortedResult() []TopHeapEntry {
	result := append([]TopHeapEntry{}, th.a...)
	sort.Slice(result, func(i, j int) bool {
		a, b := result[i], result[j]
		if a.Count != b.Count {
			return a.Count > b.Count
		}
		return a.Name < b.Name
	})
	return result
}

// heap.Interface implementation for topHeap.

func (th *topHeap) Len() int {
	return len(th.a)
}

func (th *topHeap) Less(i, j int) bool {
	a := th.a
	return a[i].Count < a[j].Count
}

func (th *topHeap) Swap(i, j int) {
	a := th.a
	a[j], a[i] = a[i], a[j]
}

func (th *topHeap) Push(x interface{}) {
	panic(fmt.Errorf("BUG: Push shouldn't be called"))
}

func (th *topHeap) Pop() interface{} {
	panic(fmt.Errorf("BUG: Pop shouldn't be called"))
}

// searchMetricNameWithCache appends metric name for the given metricID to dst
// and returns the result.
func (db *indexDB) searchMetricNameWithCache(dst []byte, metricID uint64, accountID, projectID uint32) ([]byte, error) {
	metricName := db.getMetricNameFromCache(dst, metricID)
	if len(metricName) > len(dst) {
		return metricName, nil
	}

	is := db.getIndexSearch(accountID, projectID, noDeadline)
	var err error
	dst, err = is.searchMetricName(dst, metricID)
	db.putIndexSearch(is)
	if err == nil {
		// There is no need in verifying whether the given metricID is deleted,
		// since the filtering must be performed before calling this func.
		db.putMetricNameToCache(metricID, dst)
		return dst, nil
	}
	if err != io.EOF {
		return dst, err
	}

	// Try searching in the external indexDB.
	if db.doExtDB(func(extDB *indexDB) {
		is := extDB.getIndexSearch(accountID, projectID, noDeadline)
		dst, err = is.searchMetricName(dst, metricID)
		extDB.putIndexSearch(is)
		if err == nil {
			// There is no need in verifying whether the given metricID is deleted,
			// since the filtering must be performed before calling this func.
			extDB.putMetricNameToCache(metricID, dst)
		}
	}) {
		return dst, err
	}

	// Cannot find MetricName for the given metricID. This may be the case
	// when indexDB contains incomplete set of metricID -> metricName entries
	// after a snapshot or due to unflushed entries.
	atomic.AddUint64(&db.missingMetricNamesForMetricID, 1)

	// Mark the metricID as deleted, so it will be created again when new data point
	// for the given time series will arrive.
	if err := db.deleteMetricIDs([]uint64{metricID}); err != nil {
		return dst, fmt.Errorf("cannot delete metricID for missing metricID->metricName entry; metricID=%d; error: %w", metricID, err)
	}
	return dst, io.EOF
}

// DeleteTSIDs marks as deleted all the TSIDs matching the given tfss.
//
// The caller must reset all the caches which may contain the deleted TSIDs.
//
// Returns the number of metrics deleted.
func (db *indexDB) DeleteTSIDs(tfss []*TagFilters) (int, error) {
	if len(tfss) == 0 {
		return 0, nil
	}

	// Obtain metricIDs to delete.
	tr := TimeRange{
		MinTimestamp: 0,
		MaxTimestamp: (1 << 63) - 1,
	}
	is := db.getIndexSearch(tfss[0].accountID, tfss[0].projectID, noDeadline)
	metricIDs, err := is.searchMetricIDs(tfss, tr, 2e9)
	db.putIndexSearch(is)
	if err != nil {
		return 0, err
	}
	if err := db.deleteMetricIDs(metricIDs); err != nil {
		return 0, err
	}

	// Delete TSIDs in the extDB.
	deletedCount := len(metricIDs)
	if db.doExtDB(func(extDB *indexDB) {
		var n int
		n, err = extDB.DeleteTSIDs(tfss)
		deletedCount += n
	}) {
		if err != nil {
			return deletedCount, fmt.Errorf("cannot delete tsids in extDB: %w", err)
		}
	}
	return deletedCount, nil
}

func (db *indexDB) deleteMetricIDs(metricIDs []uint64) error {
	if len(metricIDs) == 0 {
		// Nothing to delete
		return nil
	}

	// atomically add deleted metricIDs to an inmemory map.
	dmis := &uint64set.Set{}
	dmis.AddMulti(metricIDs)
	db.s.updateDeletedMetricIDs(dmis)

	// Reset TagFilters -> TSIDS cache, since it may contain deleted TSIDs.
	invalidateTagFiltersCache()

	// Reset MetricName -> TSID cache, since it may contain deleted TSIDs.
	db.s.resetAndSaveTSIDCache()

	// Store the metricIDs as deleted.
	// Make this after updating the deletedMetricIDs and resetting caches
	// in order to exclude the possibility of the inconsistent state when the deleted metricIDs
	// remain available in the tsidCache after unclean shutdown.
	// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1347
	items := getIndexItems()
	for _, metricID := range metricIDs {
		items.B = append(items.B, nsPrefixDeletedMetricID)
		items.B = encoding.MarshalUint64(items.B, metricID)
		items.Next()
	}
	err := db.tb.AddItems(items.Items)
	putIndexItems(items)
	return err
}

func (db *indexDB) loadDeletedMetricIDs() (*uint64set.Set, error) {
	is := db.getIndexSearch(0, 0, noDeadline)
	dmis, err := is.loadDeletedMetricIDs()
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}
	return dmis, nil
}

func (is *indexSearch) loadDeletedMetricIDs() (*uint64set.Set, error) {
	dmis := &uint64set.Set{}
	ts := &is.ts
	kb := &is.kb
	kb.B = append(kb.B[:0], nsPrefixDeletedMetricID)
	ts.Seek(kb.B)
	for ts.NextItem() {
		item := ts.Item
		if !bytes.HasPrefix(item, kb.B) {
			break
		}
		item = item[len(kb.B):]
		if len(item) != 8 {
			return nil, fmt.Errorf("unexpected item len; got %d bytes; want %d bytes", len(item), 8)
		}
		metricID := encoding.UnmarshalUint64(item)
		dmis.Add(metricID)
	}
	if err := ts.Error(); err != nil {
		return nil, err
	}
	return dmis, nil
}

// searchTSIDs returns sorted tsids matching the given tfss over the given tr.
func (db *indexDB) searchTSIDs(tfss []*TagFilters, tr TimeRange, maxMetrics int, deadline uint64) ([]TSID, error) {
	if len(tfss) == 0 {
		return nil, nil
	}
	if tr.MinTimestamp >= db.s.minTimestampForCompositeIndex {
		tfss = convertToCompositeTagFilterss(tfss)
	}

	tfKeyBuf := tagFiltersKeyBufPool.Get()
	defer tagFiltersKeyBufPool.Put(tfKeyBuf)

	tfKeyBuf.B = marshalTagFiltersKey(tfKeyBuf.B[:0], tfss, tr, true)
	tsids, ok := db.getFromTagFiltersCache(tfKeyBuf.B)
	if ok {
		// Fast path - tsids found in the cache.
		return tsids, nil
	}

	// Slow path - search for tsids in the db and extDB.
	accountID := tfss[0].accountID
	projectID := tfss[0].projectID
	is := db.getIndexSearch(accountID, projectID, deadline)
	localTSIDs, err := is.searchTSIDs(tfss, tr, maxMetrics)
	db.putIndexSearch(is)
	if err != nil {
		return nil, err
	}

	var extTSIDs []TSID
	if db.doExtDB(func(extDB *indexDB) {
		tfKeyExtBuf := tagFiltersKeyBufPool.Get()
		defer tagFiltersKeyBufPool.Put(tfKeyExtBuf)

		// Data in extDB cannot be changed, so use unversioned keys for tag cache.
		tfKeyExtBuf.B = marshalTagFiltersKey(tfKeyExtBuf.B[:0], tfss, tr, false)
		tsids, ok := extDB.getFromTagFiltersCache(tfKeyExtBuf.B)
		if ok {
			extTSIDs = tsids
			return
		}
		is := extDB.getIndexSearch(accountID, projectID, deadline)
		extTSIDs, err = is.searchTSIDs(tfss, tr, maxMetrics)
		extDB.putIndexSearch(is)

		sort.Slice(extTSIDs, func(i, j int) bool { return extTSIDs[i].Less(&extTSIDs[j]) })
		extDB.putToTagFiltersCache(extTSIDs, tfKeyExtBuf.B)
	}) {
		if err != nil {
			return nil, err
		}
	}

	// Merge localTSIDs with extTSIDs.
	tsids = mergeTSIDs(localTSIDs, extTSIDs)

	// Sort the found tsids, since they must be passed to TSID search
	// in the sorted order.
	sort.Slice(tsids, func(i, j int) bool { return tsids[i].Less(&tsids[j]) })

	// Store TSIDs in the cache.
	db.putToTagFiltersCache(tsids, tfKeyBuf.B)

	return tsids, err
}

var tagFiltersKeyBufPool bytesutil.ByteBufferPool

func (is *indexSearch) getTSIDByMetricName(dst *TSID, metricName []byte) error {
	dmis := is.db.s.getDeletedMetricIDs()
	ts := &is.ts
	kb := &is.kb
	kb.B = append(kb.B[:0], nsPrefixMetricNameToTSID)
	kb.B = append(kb.B, metricName...)
	kb.B = append(kb.B, kvSeparatorChar)
	ts.Seek(kb.B)
	for ts.NextItem() {
		if !bytes.HasPrefix(ts.Item, kb.B) {
			// Nothing found.
			return io.EOF
		}
		v := ts.Item[len(kb.B):]
		tail, err := dst.Unmarshal(v)
		if err != nil {
			return fmt.Errorf("cannot unmarshal TSID: %w", err)
		}
		if len(tail) > 0 {
			return fmt.Errorf("unexpected non-empty tail left after unmarshaling TSID: %X", tail)
		}
		if dmis.Len() > 0 {
			// Verify whether the dst is marked as deleted.
			if dmis.Has(dst.MetricID) {
				// The dst is deleted. Continue searching.
				continue
			}
		}
		// Found valid dst.
		return nil
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching TSID by metricName; searchPrefix %q: %w", kb.B, err)
	}
	// Nothing found
	return io.EOF
}

func (is *indexSearch) searchMetricNameWithCache(dst []byte, metricID uint64) ([]byte, error) {
	metricName := is.db.getMetricNameFromCache(dst, metricID)
	if len(metricName) > len(dst) {
		return metricName, nil
	}
	var err error
	dst, err = is.searchMetricName(dst, metricID)
	if err == nil {
		// There is no need in verifying whether the given metricID is deleted,
		// since the filtering must be performed before calling this func.
		is.db.putMetricNameToCache(metricID, dst)
		return dst, nil
	}
	return dst, err
}

func (is *indexSearch) searchMetricName(dst []byte, metricID uint64) ([]byte, error) {
	ts := &is.ts
	kb := &is.kb
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixMetricIDToMetricName)
	kb.B = encoding.MarshalUint64(kb.B, metricID)
	if err := ts.FirstItemWithPrefix(kb.B); err != nil {
		if err == io.EOF {
			return dst, err
		}
		return dst, fmt.Errorf("error when searching metricName by metricID; searchPrefix %q: %w", kb.B, err)
	}
	v := ts.Item[len(kb.B):]
	dst = append(dst, v...)
	return dst, nil
}

func mergeTSIDs(a, b []TSID) []TSID {
	if len(b) > len(a) {
		a, b = b, a
	}
	if len(b) == 0 {
		return a
	}
	m := make(map[uint64]TSID, len(a))
	for i := range a {
		tsid := &a[i]
		m[tsid.MetricID] = *tsid
	}
	for i := range b {
		tsid := &b[i]
		m[tsid.MetricID] = *tsid
	}

	tsids := make([]TSID, 0, len(m))
	for _, tsid := range m {
		tsids = append(tsids, tsid)
	}
	return tsids
}

func (is *indexSearch) containsTimeRange(tr TimeRange) (bool, error) {
	ts := &is.ts
	kb := &is.kb

	// Verify whether the tr.MinTimestamp is included into `ts` or is smaller than the minimum date stored in `ts`.
	// Do not check whether tr.MaxTimestamp is included into `ts` or is bigger than the max date stored in `ts` for performance reasons.
	// This means that containsTimeRange() can return true if `tr` is located below the min date stored in `ts`.
	// This is OK, since this case isn't encountered too much in practice.
	// The main practical case allows skipping searching in prev indexdb (`ts`) when `tr`
	// is located above the max date stored there.
	minDate := uint64(tr.MinTimestamp) / msecPerDay
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixDateToMetricID)
	prefix := kb.B
	kb.B = encoding.MarshalUint64(kb.B, minDate)
	ts.Seek(kb.B)
	if !ts.NextItem() {
		if err := ts.Error(); err != nil {
			return false, fmt.Errorf("error when searching for minDate=%d, prefix %q: %w", minDate, kb.B, err)
		}
		return false, nil
	}
	if !bytes.HasPrefix(ts.Item, prefix) {
		// minDate exceeds max date from ts.
		return false, nil
	}
	return true, nil
}

func (is *indexSearch) searchTSIDs(tfss []*TagFilters, tr TimeRange, maxMetrics int) ([]TSID, error) {
	ok, err := is.containsTimeRange(tr)
	if err != nil {
		return nil, err
	}
	if !ok {
		// Fast path - the index doesn't contain data for the given tr.
		return nil, nil
	}
	metricIDs, err := is.searchMetricIDs(tfss, tr, maxMetrics)
	if err != nil {
		return nil, err
	}
	if len(metricIDs) == 0 {
		// Nothing found.
		return nil, nil
	}

	// Obtain TSID values for the given metricIDs.
	tsids := make([]TSID, len(metricIDs))
	i := 0
	for loopsPaceLimiter, metricID := range metricIDs {
		if loopsPaceLimiter&paceLimiterSlowIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return nil, err
			}
		}
		// Try obtaining TSIDs from MetricID->TSID cache. This is much faster
		// than scanning the mergeset if it contains a lot of metricIDs.
		tsid := &tsids[i]
		err := is.db.getFromMetricIDCache(tsid, metricID)
		if err == nil {
			// Fast path - the tsid for metricID is found in cache.
			i++
			continue
		}
		if err != io.EOF {
			return nil, err
		}
		if err := is.getTSIDByMetricID(tsid, metricID); err != nil {
			if err == io.EOF {
				// Cannot find TSID for the given metricID.
				// This may be the case on incomplete indexDB
				// due to snapshot or due to unflushed entries.
				// Just increment errors counter and skip it.
				atomic.AddUint64(&is.db.missingTSIDsForMetricID, 1)
				continue
			}
			return nil, fmt.Errorf("cannot find tsid %d out of %d for metricID %d: %w", i, len(metricIDs), metricID, err)
		}
		is.db.putToMetricIDCache(metricID, tsid)
		i++
	}
	tsids = tsids[:i]

	// Do not sort the found tsids, since they will be sorted later.
	return tsids, nil
}

func (is *indexSearch) getTSIDByMetricID(dst *TSID, metricID uint64) error {
	// There is no need in checking for deleted metricIDs here, since they
	// must be checked by the caller.
	ts := &is.ts
	kb := &is.kb
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixMetricIDToTSID)
	kb.B = encoding.MarshalUint64(kb.B, metricID)
	if err := ts.FirstItemWithPrefix(kb.B); err != nil {
		if err == io.EOF {
			return err
		}
		return fmt.Errorf("error when searching TSID by metricID; searchPrefix %q: %w", kb.B, err)
	}
	v := ts.Item[len(kb.B):]
	tail, err := dst.Unmarshal(v)
	if err != nil {
		return fmt.Errorf("cannot unmarshal TSID=%X: %w", v, err)
	}
	if len(tail) > 0 {
		return fmt.Errorf("unexpected non-zero tail left after unmarshaling TSID: %X", tail)
	}
	return nil
}

// updateMetricIDsByMetricNameMatch matches metricName values for the given srcMetricIDs against tfs
// and adds matching metrics to metricIDs.
func (is *indexSearch) updateMetricIDsByMetricNameMatch(metricIDs, srcMetricIDs *uint64set.Set, tfs []*tagFilter) error {
	// sort srcMetricIDs in order to speed up Seek below.
	sortedMetricIDs := srcMetricIDs.AppendTo(nil)

	kb := &is.kb
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	tfs = removeCompositeTagFilters(tfs, kb.B)

	metricName := kbPool.Get()
	defer kbPool.Put(metricName)
	mn := GetMetricName()
	defer PutMetricName(mn)
	for loopsPaceLimiter, metricID := range sortedMetricIDs {
		if loopsPaceLimiter&paceLimiterSlowIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return err
			}
		}
		var err error
		metricName.B, err = is.searchMetricNameWithCache(metricName.B[:0], metricID)
		if err != nil {
			if err == io.EOF {
				// It is likely the metricID->metricName entry didn't propagate to inverted index yet.
				// Skip this metricID for now.
				continue
			}
			return fmt.Errorf("cannot find metricName by metricID %d: %w", metricID, err)
		}
		if err := mn.Unmarshal(metricName.B); err != nil {
			return fmt.Errorf("cannot unmarshal metricName %q: %w", metricName.B, err)
		}

		// Match the mn against tfs.
		ok, err := matchTagFilters(mn, tfs, &is.kb)
		if err != nil {
			return fmt.Errorf("cannot match MetricName %s against tagFilters: %w", mn, err)
		}
		if !ok {
			continue
		}
		metricIDs.Add(metricID)
	}
	return nil
}

func removeCompositeTagFilters(tfs []*tagFilter, prefix []byte) []*tagFilter {
	if !hasCompositeTagFilters(tfs, prefix) {
		return tfs
	}
	var tagKey []byte
	var name []byte
	tfsNew := make([]*tagFilter, 0, len(tfs)+1)
	for _, tf := range tfs {
		if !bytes.HasPrefix(tf.prefix, prefix) {
			tfsNew = append(tfsNew, tf)
			continue
		}
		suffix := tf.prefix[len(prefix):]
		var err error
		_, tagKey, err = unmarshalTagValue(tagKey[:0], suffix)
		if err != nil {
			logger.Panicf("BUG: cannot unmarshal tag key from suffix=%q: %s", suffix, err)
		}
		if len(tagKey) == 0 || tagKey[0] != compositeTagKeyPrefix {
			tfsNew = append(tfsNew, tf)
			continue
		}
		tagKey = tagKey[1:]
		var nameLen uint64
		tagKey, nameLen, err = encoding.UnmarshalVarUint64(tagKey)
		if err != nil {
			logger.Panicf("BUG: cannot unmarshal nameLen from tagKey %q: %s", tagKey, err)
		}
		if nameLen == 0 {
			logger.Panicf("BUG: nameLen must be greater than 0")
		}
		if uint64(len(tagKey)) < nameLen {
			logger.Panicf("BUG: expecting at %d bytes for name in tagKey=%q; got %d bytes", nameLen, tagKey, len(tagKey))
		}
		name = append(name[:0], tagKey[:nameLen]...)
		tagKey = tagKey[nameLen:]
		var tfNew tagFilter
		if err := tfNew.Init(prefix, tagKey, tf.value, tf.isNegative, tf.isRegexp); err != nil {
			logger.Panicf("BUG: cannot initialize {%s=%q} filter: %s", tagKey, tf.value, err)
		}
		tfsNew = append(tfsNew, &tfNew)
	}
	if len(name) > 0 {
		var tfNew tagFilter
		if err := tfNew.Init(prefix, nil, name, false, false); err != nil {
			logger.Panicf("BUG: unexpected error when initializing {__name__=%q} filter: %s", name, err)
		}
		tfsNew = append(tfsNew, &tfNew)
	}
	return tfsNew
}

func hasCompositeTagFilters(tfs []*tagFilter, prefix []byte) bool {
	var tagKey []byte
	for _, tf := range tfs {
		if !bytes.HasPrefix(tf.prefix, prefix) {
			continue
		}
		suffix := tf.prefix[len(prefix):]
		var err error
		_, tagKey, err = unmarshalTagValue(tagKey[:0], suffix)
		if err != nil {
			logger.Panicf("BUG: cannot unmarshal tag key from suffix=%q: %s", suffix, err)
		}
		if len(tagKey) > 0 && tagKey[0] == compositeTagKeyPrefix {
			return true
		}
	}
	return false
}

func matchTagFilters(mn *MetricName, tfs []*tagFilter, kb *bytesutil.ByteBuffer) (bool, error) {
	kb.B = marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs, mn.AccountID, mn.ProjectID)
	for i, tf := range tfs {
		if bytes.Equal(tf.key, graphiteReverseTagKey) {
			// Skip artificial tag filter for Graphite-like metric names with dots,
			// since mn doesn't contain the corresponding tag.
			continue
		}
		if len(tf.key) == 0 || string(tf.key) == "__graphite__" {
			// Match against mn.MetricGroup.
			b := marshalTagValue(kb.B, nil)
			b = marshalTagValue(b, mn.MetricGroup)
			kb.B = b[:len(kb.B)]
			ok, err := tf.match(b)
			if err != nil {
				return false, fmt.Errorf("cannot match MetricGroup %q with tagFilter %s: %w", mn.MetricGroup, tf, err)
			}
			if !ok {
				// Move failed tf to start.
				// This should reduce the amount of useless work for the next mn.
				if i > 0 {
					tfs[0], tfs[i] = tfs[i], tfs[0]
				}
				return false, nil
			}
			continue
		}
		// Search for matching tag name.
		tagMatched := false
		tagSeen := false
		for _, tag := range mn.Tags {
			if string(tag.Key) != string(tf.key) {
				continue
			}

			// Found the matching tag name. Match the value.
			tagSeen = true
			b := tag.Marshal(kb.B)
			kb.B = b[:len(kb.B)]
			ok, err := tf.match(b)
			if err != nil {
				return false, fmt.Errorf("cannot match tag %q with tagFilter %s: %w", tag, tf, err)
			}
			if !ok {
				// Move failed tf to start.
				// This should reduce the amount of useless work for the next mn.
				if i > 0 {
					tfs[0], tfs[i] = tfs[i], tfs[0]
				}
				return false, nil
			}
			tagMatched = true
			break
		}
		if !tagSeen && (!tf.isNegative && tf.isEmptyMatch || tf.isNegative && !tf.isEmptyMatch) {
			// tf contains positive empty-match filter for non-existing tag key, i.e.
			// {non_existing_tag_key=~"foobar|"}
			//
			// OR
			//
			// tf contains negative filter for non-exsisting tag key
			// and this filter doesn't match empty string, i.e. {non_existing_tag_key!="foobar"}
			// Such filter matches anything.
			//
			// Note that the filter `{non_existing_tag_key!~"|foobar"}` shouldn't match anything,
			// since it is expected that it matches non-empty `non_existing_tag_key`.
			// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/546 and
			// https://github.com/VictoriaMetrics/VictoriaMetrics/issues/2255 for details.
			continue
		}
		if tagMatched {
			// tf matches mn. Go to the next tf.
			continue
		}
		// Matching tag name wasn't found.
		// Move failed tf to start.
		// This should reduce the amount of useless work for the next mn.
		if i > 0 {
			tfs[0], tfs[i] = tfs[i], tfs[0]
		}
		return false, nil
	}
	return true, nil
}

func (is *indexSearch) searchMetricIDs(tfss []*TagFilters, tr TimeRange, maxMetrics int) ([]uint64, error) {
	metricIDs, err := is.searchMetricIDsInternal(tfss, tr, maxMetrics)
	if err != nil {
		return nil, err
	}
	if metricIDs.Len() == 0 {
		// Nothing found
		return nil, nil
	}

	sortedMetricIDs := metricIDs.AppendTo(nil)

	// Filter out deleted metricIDs.
	dmis := is.db.s.getDeletedMetricIDs()
	if dmis.Len() > 0 {
		metricIDsFiltered := sortedMetricIDs[:0]
		for _, metricID := range sortedMetricIDs {
			if !dmis.Has(metricID) {
				metricIDsFiltered = append(metricIDsFiltered, metricID)
			}
		}
		sortedMetricIDs = metricIDsFiltered
	}

	return sortedMetricIDs, nil
}

func (is *indexSearch) searchMetricIDsInternal(tfss []*TagFilters, tr TimeRange, maxMetrics int) (*uint64set.Set, error) {
	metricIDs := &uint64set.Set{}
	for _, tfs := range tfss {
		if len(tfs.tfs) == 0 {
			// An empty filters must be equivalent to `{__name__!=""}`
			tfs = NewTagFilters(tfs.accountID, tfs.projectID)
			if err := tfs.Add(nil, nil, true, false); err != nil {
				logger.Panicf(`BUG: cannot add {__name__!=""} filter: %s`, err)
			}
		}
		if err := is.updateMetricIDsForTagFilters(metricIDs, tfs, tr, maxMetrics+1); err != nil {
			return nil, err
		}
		if metricIDs.Len() > maxMetrics {
			return nil, fmt.Errorf("the number of matching unique timeseries exceeds %d; either narrow down the search or increase -search.maxUniqueTimeseries", maxMetrics)
		}
	}
	return metricIDs, nil
}

func (is *indexSearch) updateMetricIDsForTagFilters(metricIDs *uint64set.Set, tfs *TagFilters, tr TimeRange, maxMetrics int) error {
	err := is.tryUpdatingMetricIDsForDateRange(metricIDs, tfs, tr, maxMetrics)
	if err == nil {
		// Fast path: found metricIDs by date range.
		return nil
	}
	if !errors.Is(err, errFallbackToGlobalSearch) {
		return err
	}

	// Slow path - fall back to search in the global inverted index.
	atomic.AddUint64(&is.db.globalSearchCalls, 1)
	m, err := is.getMetricIDsForDateAndFilters(0, tfs, maxMetrics)
	if err != nil {
		return err
	}
	metricIDs.UnionMayOwn(m)
	return nil
}

func (is *indexSearch) getMetricIDsForTagFilter(tf *tagFilter, maxMetrics int, maxLoopsCount int64) (*uint64set.Set, int64, error) {
	if tf.isNegative {
		logger.Panicf("BUG: isNegative must be false")
	}
	metricIDs := &uint64set.Set{}
	if len(tf.orSuffixes) > 0 {
		// Fast path for orSuffixes - seek for rows for each value from orSuffixes.
		loopsCount, err := is.updateMetricIDsForOrSuffixes(tf, metricIDs, maxMetrics, maxLoopsCount)
		if err != nil {
			return nil, loopsCount, fmt.Errorf("error when searching for metricIDs for tagFilter in fast path: %w; tagFilter=%s", err, tf)
		}
		return metricIDs, loopsCount, nil
	}

	// Slow path - scan for all the rows with the given prefix.
	loopsCount, err := is.getMetricIDsForTagFilterSlow(tf, metricIDs.Add, maxLoopsCount)
	if err != nil {
		return nil, loopsCount, fmt.Errorf("error when searching for metricIDs for tagFilter in slow path: %w; tagFilter=%s", err, tf)
	}
	return metricIDs, loopsCount, nil
}

var errTooManyLoops = fmt.Errorf("too many loops is needed for applying this filter")

func (is *indexSearch) getMetricIDsForTagFilterSlow(tf *tagFilter, f func(metricID uint64), maxLoopsCount int64) (int64, error) {
	if len(tf.orSuffixes) > 0 {
		logger.Panicf("BUG: the getMetricIDsForTagFilterSlow must be called only for empty tf.orSuffixes; got %s", tf.orSuffixes)
	}

	// Scan all the rows with tf.prefix and call f on every tf match.
	ts := &is.ts
	kb := &is.kb
	mp := &is.mp
	mp.Reset()
	var prevMatchingSuffix []byte
	var prevMatch bool
	var loopsCount int64
	loopsPaceLimiter := 0
	prefix := tf.prefix
	ts.Seek(prefix)
	for ts.NextItem() {
		if loopsPaceLimiter&paceLimiterMediumIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return loopsCount, err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			return loopsCount, nil
		}
		tail := item[len(prefix):]
		n := bytes.IndexByte(tail, tagSeparatorChar)
		if n < 0 {
			return loopsCount, fmt.Errorf("invalid tag->metricIDs line %q: cannot find tagSeparatorChar=%d", item, tagSeparatorChar)
		}
		suffix := tail[:n+1]
		tail = tail[n+1:]
		if err := mp.InitOnlyTail(item, tail); err != nil {
			return loopsCount, err
		}
		mp.ParseMetricIDs()
		loopsCount += int64(mp.MetricIDsLen())
		if loopsCount > maxLoopsCount {
			return loopsCount, errTooManyLoops
		}
		if prevMatch && string(suffix) == string(prevMatchingSuffix) {
			// Fast path: the same tag value found.
			// There is no need in checking it again with potentially
			// slow tf.matchSuffix, which may call regexp.
			for _, metricID := range mp.MetricIDs {
				f(metricID)
			}
			continue
		}
		// Slow path: need tf.matchSuffix call.
		ok, err := tf.matchSuffix(suffix)
		// Assume that tf.matchSuffix call needs 10x more time than a single metric scan iteration.
		loopsCount += 10 * int64(tf.matchCost)
		if err != nil {
			return loopsCount, fmt.Errorf("error when matching %s against suffix %q: %w", tf, suffix, err)
		}
		if !ok {
			prevMatch = false
			if mp.MetricIDsLen() < maxMetricIDsPerRow/2 {
				// If the current row contains non-full metricIDs list,
				// then it is likely the next row contains the next tag value.
				// So skip seeking for the next tag value, since it will be slower than just ts.NextItem call.
				continue
			}
			// Optimization: skip all the metricIDs for the given tag value
			kb.B = append(kb.B[:0], item[:len(item)-len(tail)]...)
			// The last char in kb.B must be tagSeparatorChar. Just increment it
			// in order to jump to the next tag value.
			if len(kb.B) == 0 || kb.B[len(kb.B)-1] != tagSeparatorChar || tagSeparatorChar >= 0xff {
				return loopsCount, fmt.Errorf("data corruption: the last char in k=%X must be %X", kb.B, tagSeparatorChar)
			}
			kb.B[len(kb.B)-1]++
			ts.Seek(kb.B)
			// Assume that a seek cost is equivalent to 1000 ordinary loops.
			loopsCount += 1000
			continue
		}
		prevMatch = true
		prevMatchingSuffix = append(prevMatchingSuffix[:0], suffix...)
		for _, metricID := range mp.MetricIDs {
			f(metricID)
		}
	}
	if err := ts.Error(); err != nil {
		return loopsCount, fmt.Errorf("error when searching for tag filter prefix %q: %w", prefix, err)
	}
	return loopsCount, nil
}

func (is *indexSearch) updateMetricIDsForOrSuffixes(tf *tagFilter, metricIDs *uint64set.Set, maxMetrics int, maxLoopsCount int64) (int64, error) {
	if tf.isNegative {
		logger.Panicf("BUG: isNegative must be false")
	}
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	var loopsCount int64
	for _, orSuffix := range tf.orSuffixes {
		kb.B = append(kb.B[:0], tf.prefix...)
		kb.B = append(kb.B, orSuffix...)
		kb.B = append(kb.B, tagSeparatorChar)
		lc, err := is.updateMetricIDsForOrSuffix(kb.B, metricIDs, maxMetrics, maxLoopsCount-loopsCount)
		loopsCount += lc
		if err != nil {
			return loopsCount, err
		}
		if metricIDs.Len() >= maxMetrics {
			return loopsCount, nil
		}
	}
	return loopsCount, nil
}

func (is *indexSearch) updateMetricIDsForOrSuffix(prefix []byte, metricIDs *uint64set.Set, maxMetrics int, maxLoopsCount int64) (int64, error) {
	ts := &is.ts
	mp := &is.mp
	mp.Reset()
	var loopsCount int64
	loopsPaceLimiter := 0
	ts.Seek(prefix)
	for metricIDs.Len() < maxMetrics && ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return loopsCount, err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			return loopsCount, nil
		}
		if err := mp.InitOnlyTail(item, item[len(prefix):]); err != nil {
			return loopsCount, err
		}
		loopsCount += int64(mp.MetricIDsLen())
		if loopsCount > maxLoopsCount {
			return loopsCount, errTooManyLoops
		}
		mp.ParseMetricIDs()
		metricIDs.AddMulti(mp.MetricIDs)
	}
	if err := ts.Error(); err != nil {
		return loopsCount, fmt.Errorf("error when searching for tag filter prefix %q: %w", prefix, err)
	}
	return loopsCount, nil
}

var errFallbackToGlobalSearch = errors.New("fall back from per-day index search to global index search")

const maxDaysForPerDaySearch = 40

func (is *indexSearch) tryUpdatingMetricIDsForDateRange(metricIDs *uint64set.Set, tfs *TagFilters, tr TimeRange, maxMetrics int) error {
	atomic.AddUint64(&is.db.dateRangeSearchCalls, 1)
	minDate := uint64(tr.MinTimestamp) / msecPerDay
	maxDate := uint64(tr.MaxTimestamp) / msecPerDay
	if minDate > maxDate || maxDate-minDate > maxDaysForPerDaySearch {
		// Too much dates must be covered. Give up, since it may be slow.
		return errFallbackToGlobalSearch
	}
	if minDate == maxDate {
		// Fast path - query only a single date.
		m, err := is.getMetricIDsForDateAndFilters(minDate, tfs, maxMetrics)
		if err != nil {
			return err
		}
		metricIDs.UnionMayOwn(m)
		atomic.AddUint64(&is.db.dateRangeSearchHits, 1)
		return nil
	}

	// Slower path - search for metricIDs for each day in parallel.
	wg := getWaitGroup()
	var errGlobal error
	var mu sync.Mutex // protects metricIDs + errGlobal vars from concurrent access below
	for minDate <= maxDate {
		wg.Add(1)
		go func(date uint64) {
			defer wg.Done()
			isLocal := is.db.getIndexSearch(is.accountID, is.projectID, is.deadline)
			m, err := isLocal.getMetricIDsForDateAndFilters(date, tfs, maxMetrics)
			is.db.putIndexSearch(isLocal)
			mu.Lock()
			defer mu.Unlock()
			if errGlobal != nil {
				return
			}
			if err != nil {
				dateStr := time.Unix(int64(date*24*3600), 0)
				errGlobal = fmt.Errorf("cannot search for metricIDs at %s: %w", dateStr, err)
				return
			}
			if metricIDs.Len() < maxMetrics {
				metricIDs.UnionMayOwn(m)
			}
		}(minDate)
		minDate++
	}
	wg.Wait()
	putWaitGroup(wg)
	if errGlobal != nil {
		return errGlobal
	}
	atomic.AddUint64(&is.db.dateRangeSearchHits, 1)
	return nil
}

func (is *indexSearch) getMetricIDsForDateAndFilters(date uint64, tfs *TagFilters, maxMetrics int) (*uint64set.Set, error) {
	// Sort tfs by loopsCount needed for performing each filter.
	// This stats is usually collected from the previous queries.
	// This way we limit the amount of work below by applying fast filters at first.
	type tagFilterWithWeight struct {
		tf               *tagFilter
		loopsCount       int64
		filterLoopsCount int64
	}
	tfws := make([]tagFilterWithWeight, len(tfs.tfs))
	currentTime := fasttime.UnixTimestamp()
	for i := range tfs.tfs {
		tf := &tfs.tfs[i]
		loopsCount, filterLoopsCount, timestamp := is.getLoopsCountAndTimestampForDateFilter(date, tf)
		if currentTime > timestamp+3600 {
			// Update stats once per hour for relatively fast tag filters.
			// There is no need in spending CPU resources on updating stats for heavy tag filters.
			if loopsCount <= 10e6 {
				loopsCount = 0
			}
			if filterLoopsCount <= 10e6 {
				filterLoopsCount = 0
			}
		}
		tfws[i] = tagFilterWithWeight{
			tf:               tf,
			loopsCount:       loopsCount,
			filterLoopsCount: filterLoopsCount,
		}
	}
	sort.Slice(tfws, func(i, j int) bool {
		a, b := &tfws[i], &tfws[j]
		if a.loopsCount != b.loopsCount {
			return a.loopsCount < b.loopsCount
		}
		return a.tf.Less(b.tf)
	})
	getFirstPositiveLoopsCount := func(tfws []tagFilterWithWeight) int64 {
		for i := range tfws {
			if n := tfws[i].loopsCount; n > 0 {
				return n
			}
		}
		return int64Max
	}
	storeLoopsCount := func(tfw *tagFilterWithWeight, loopsCount int64) {
		if loopsCount != tfw.loopsCount {
			tfw.loopsCount = loopsCount
			is.storeLoopsCountForDateFilter(date, tfw.tf, tfw.loopsCount, tfw.filterLoopsCount)
		}
	}

	// Populate metricIDs for the first non-negative filter with the cost smaller than maxLoopsCount.
	var metricIDs *uint64set.Set
	tfwsRemaining := tfws[:0]
	maxDateMetrics := intMax
	if maxMetrics < intMax/50 {
		maxDateMetrics = maxMetrics * 50
	}
	for i, tfw := range tfws {
		tf := tfw.tf
		if tf.isNegative || tf.isEmptyMatch {
			tfwsRemaining = append(tfwsRemaining, tfw)
			continue
		}
		maxLoopsCount := getFirstPositiveLoopsCount(tfws[i+1:])
		m, loopsCount, err := is.getMetricIDsForDateTagFilter(tf, date, tfs.commonPrefix, maxDateMetrics, maxLoopsCount)
		if err != nil {
			if errors.Is(err, errTooManyLoops) {
				// The tf took too many loops compared to the next filter. Postpone applying this filter.
				storeLoopsCount(&tfw, 2*loopsCount)
				tfwsRemaining = append(tfwsRemaining, tfw)
				continue
			}
			// Move failing filter to the end of filter list.
			storeLoopsCount(&tfw, int64Max)
			return nil, err
		}
		if m.Len() >= maxDateMetrics {
			// Too many time series found by a single tag filter. Move the filter to the end of list.
			storeLoopsCount(&tfw, int64Max-1)
			tfwsRemaining = append(tfwsRemaining, tfw)
			continue
		}
		storeLoopsCount(&tfw, loopsCount)
		metricIDs = m
		tfwsRemaining = append(tfwsRemaining, tfws[i+1:]...)
		break
	}
	tfws = tfwsRemaining

	if metricIDs == nil {
		// All the filters in tfs are negative or match too many time series.
		// Populate all the metricIDs for the given (date),
		// so later they can be filtered out with negative filters.
		m, err := is.getMetricIDsForDate(date, maxDateMetrics)
		if err != nil {
			return nil, fmt.Errorf("cannot obtain all the metricIDs: %w", err)
		}
		if m.Len() >= maxDateMetrics {
			// Too many time series found for the given (date). Fall back to global search.
			return nil, errFallbackToGlobalSearch
		}
		metricIDs = m
	}

	sort.Slice(tfws, func(i, j int) bool {
		a, b := &tfws[i], &tfws[j]
		if a.filterLoopsCount != b.filterLoopsCount {
			return a.filterLoopsCount < b.filterLoopsCount
		}
		return a.tf.Less(b.tf)
	})
	getFirstPositiveFilterLoopsCount := func(tfws []tagFilterWithWeight) int64 {
		for i := range tfws {
			if n := tfws[i].filterLoopsCount; n > 0 {
				return n
			}
		}
		return int64Max
	}
	storeFilterLoopsCount := func(tfw *tagFilterWithWeight, filterLoopsCount int64) {
		if filterLoopsCount != tfw.filterLoopsCount {
			is.storeLoopsCountForDateFilter(date, tfw.tf, tfw.loopsCount, filterLoopsCount)
		}
	}

	// Intersect metricIDs with the rest of filters.
	//
	// Do not run these tag filters in parallel, since this may result in CPU and RAM waste
	// when the intial tag filters significantly reduce the number of found metricIDs,
	// so the remaining filters could be performed via much faster metricName matching instead
	// of slow selecting of matching metricIDs.
	var tfsPostponed []*tagFilter
	for i, tfw := range tfws {
		tf := tfw.tf
		metricIDsLen := metricIDs.Len()
		if metricIDsLen == 0 {
			// There is no need in applying the remaining filters to an empty set.
			break
		}
		if tfw.filterLoopsCount > int64(metricIDsLen)*loopsCountPerMetricNameMatch {
			// It should be faster performing metricName match on the remaining filters
			// instead of scanning big number of entries in the inverted index for these filters.
			for _, tfw := range tfws[i:] {
				tfsPostponed = append(tfsPostponed, tfw.tf)
			}
			break
		}
		maxLoopsCount := getFirstPositiveFilterLoopsCount(tfws[i+1:])
		if maxLoopsCount == int64Max {
			maxLoopsCount = int64(metricIDsLen) * loopsCountPerMetricNameMatch
		}
		m, filterLoopsCount, err := is.getMetricIDsForDateTagFilter(tf, date, tfs.commonPrefix, intMax, maxLoopsCount)
		if err != nil {
			if errors.Is(err, errTooManyLoops) {
				// Postpone tf, since it took more loops than the next filter may need.
				storeFilterLoopsCount(&tfw, 2*filterLoopsCount)
				tfsPostponed = append(tfsPostponed, tf)
				continue
			}
			// Move failing tf to the end of filter list
			storeFilterLoopsCount(&tfw, int64Max)
			return nil, err
		}
		storeFilterLoopsCount(&tfw, filterLoopsCount)
		if tf.isNegative || tf.isEmptyMatch {
			metricIDs.Subtract(m)
		} else {
			metricIDs.Intersect(m)
		}
	}
	if metricIDs.Len() == 0 {
		// There is no need in applying tfsPostponed, since the result is empty.
		return nil, nil
	}
	if len(tfsPostponed) > 0 {
		// Apply the postponed filters via metricName match.
		var m uint64set.Set
		if err := is.updateMetricIDsByMetricNameMatch(&m, metricIDs, tfsPostponed); err != nil {
			return nil, err
		}
		return &m, nil
	}
	return metricIDs, nil
}

const (
	intMax   = int((^uint(0)) >> 1)
	int64Max = int64((1 << 63) - 1)
)

func (is *indexSearch) storeDateMetricID(date, metricID uint64, mn *MetricName) error {
	ii := getIndexItems()
	defer putIndexItems(ii)

	ii.B = is.marshalCommonPrefix(ii.B, nsPrefixDateToMetricID)
	ii.B = encoding.MarshalUint64(ii.B, date)
	ii.B = encoding.MarshalUint64(ii.B, metricID)
	ii.Next()

	// Create per-day inverted index entries for metricID.
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixDateTagToMetricIDs)
	kb.B = encoding.MarshalUint64(kb.B, date)
	ii.registerTagIndexes(kb.B, mn, metricID)
	if err := is.db.tb.AddItems(ii.Items); err != nil {
		return fmt.Errorf("cannot add per-day entires for metricID %d: %w", metricID, err)
	}
	return nil
}

func (ii *indexItems) registerTagIndexes(prefix []byte, mn *MetricName, metricID uint64) {
	// Add index entry for MetricGroup -> MetricID
	ii.B = append(ii.B, prefix...)
	ii.B = marshalTagValue(ii.B, nil)
	ii.B = marshalTagValue(ii.B, mn.MetricGroup)
	ii.B = encoding.MarshalUint64(ii.B, metricID)
	ii.Next()
	ii.addReverseMetricGroupIfNeeded(prefix, mn, metricID)

	// Add index entries for tags: tag -> MetricID
	for _, tag := range mn.Tags {
		ii.B = append(ii.B, prefix...)
		ii.B = tag.Marshal(ii.B)
		ii.B = encoding.MarshalUint64(ii.B, metricID)
		ii.Next()
	}

	// Add index entries for composite tags: MetricGroup+tag -> MetricID
	compositeKey := kbPool.Get()
	for _, tag := range mn.Tags {
		compositeKey.B = marshalCompositeTagKey(compositeKey.B[:0], mn.MetricGroup, tag.Key)
		ii.B = append(ii.B, prefix...)
		ii.B = marshalTagValue(ii.B, compositeKey.B)
		ii.B = marshalTagValue(ii.B, tag.Value)
		ii.B = encoding.MarshalUint64(ii.B, metricID)
		ii.Next()
	}
	kbPool.Put(compositeKey)
}

func (ii *indexItems) addReverseMetricGroupIfNeeded(prefix []byte, mn *MetricName, metricID uint64) {
	if bytes.IndexByte(mn.MetricGroup, '.') < 0 {
		// The reverse metric group is needed only for Graphite-like metrics with points.
		return
	}
	// This is most likely a Graphite metric like 'foo.bar.baz'.
	// Store reverse metric name 'zab.rab.oof' in order to speed up search for '*.bar.baz'
	// when the Graphite wildcard has a suffix matching small number of time series.
	ii.B = append(ii.B, prefix...)
	ii.B = marshalTagValue(ii.B, graphiteReverseTagKey)
	revBuf := kbPool.Get()
	revBuf.B = reverseBytes(revBuf.B[:0], mn.MetricGroup)
	ii.B = marshalTagValue(ii.B, revBuf.B)
	kbPool.Put(revBuf)
	ii.B = encoding.MarshalUint64(ii.B, metricID)
	ii.Next()
}

func isArtificialTagKey(key []byte) bool {
	if bytes.Equal(key, graphiteReverseTagKey) {
		return true
	}
	if len(key) > 0 && key[0] == compositeTagKeyPrefix {
		return true
	}
	return false
}

// The tag key for reverse metric name used for speeding up searching
// for Graphite wildcards with suffix matching small number of time series,
// i.e. '*.bar.baz'.
//
// It is expected that the given key isn't be used by users.
var graphiteReverseTagKey = []byte("\xff")

// The prefix for composite tag, which is used for speeding up searching
// for composite filters, which contain `{__name__="<metric_name>"}` filter.
//
// It is expected that the given prefix isn't used by users.
const compositeTagKeyPrefix = '\xfe'

func marshalCompositeTagKey(dst, name, key []byte) []byte {
	dst = append(dst, compositeTagKeyPrefix)
	dst = encoding.MarshalVarUint64(dst, uint64(len(name)))
	dst = append(dst, name...)
	dst = append(dst, key...)
	return dst
}

func reverseBytes(dst, src []byte) []byte {
	for i := len(src) - 1; i >= 0; i-- {
		dst = append(dst, src[i])
	}
	return dst
}

func (is *indexSearch) hasDateMetricID(date, metricID uint64) (bool, error) {
	ts := &is.ts
	kb := &is.kb
	kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixDateToMetricID)
	kb.B = encoding.MarshalUint64(kb.B, date)
	kb.B = encoding.MarshalUint64(kb.B, metricID)
	if err := ts.FirstItemWithPrefix(kb.B); err != nil {
		if err == io.EOF {
			return false, nil
		}
		return false, fmt.Errorf("error when searching for (date=%d, metricID=%d) entry: %w", date, metricID, err)
	}
	if string(ts.Item) != string(kb.B) {
		return false, fmt.Errorf("unexpected entry for (date=%d, metricID=%d); got %q; want %q", date, metricID, ts.Item, kb.B)
	}
	return true, nil
}

func (is *indexSearch) getMetricIDsForDateTagFilter(tf *tagFilter, date uint64, commonPrefix []byte, maxMetrics int, maxLoopsCount int64) (*uint64set.Set, int64, error) {
	if !bytes.HasPrefix(tf.prefix, commonPrefix) {
		logger.Panicf("BUG: unexpected tf.prefix %q; must start with commonPrefix %q", tf.prefix, commonPrefix)
	}
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	if date != 0 {
		// Use per-date search.
		kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixDateTagToMetricIDs)
		kb.B = encoding.MarshalUint64(kb.B, date)
	} else {
		// Use global search if date isn't set.
		kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	}
	prefix := kb.B
	kb.B = append(kb.B, tf.prefix[len(commonPrefix):]...)
	tfNew := *tf
	tfNew.isNegative = false // isNegative for the original tf is handled by the caller.
	tfNew.prefix = kb.B
	metricIDs, loopsCount, err := is.getMetricIDsForTagFilter(&tfNew, maxMetrics, maxLoopsCount)
	if err != nil {
		return nil, loopsCount, err
	}
	if tf.isNegative || !tf.isEmptyMatch {
		return metricIDs, loopsCount, nil
	}
	// The tag filter, which matches empty label such as {foo=~"bar|"}
	// Convert it to negative filter, which matches {foo=~".+",foo!~"bar|"}.
	// This fixes https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1601
	// See also https://github.com/VictoriaMetrics/VictoriaMetrics/issues/395
	maxLoopsCount -= loopsCount
	tfNew = tagFilter{}
	if err := tfNew.Init(prefix, tf.key, []byte(".+"), false, true); err != nil {
		logger.Panicf(`BUG: cannot init tag filter: {%q=~".+"}: %s`, tf.key, err)
	}
	m, lc, err := is.getMetricIDsForTagFilter(&tfNew, maxMetrics, maxLoopsCount)
	loopsCount += lc
	if err != nil {
		return nil, loopsCount, err
	}
	m.Subtract(metricIDs)
	return m, loopsCount, nil
}

func (is *indexSearch) getLoopsCountAndTimestampForDateFilter(date uint64, tf *tagFilter) (int64, int64, uint64) {
	is.kb.B = appendDateTagFilterCacheKey(is.kb.B[:0], is.db.name, date, tf, is.accountID, is.projectID)
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	kb.B = is.db.loopsPerDateTagFilterCache.Get(kb.B[:0], is.kb.B)
	if len(kb.B) != 3*8 {
		return 0, 0, 0
	}
	loopsCount := encoding.UnmarshalInt64(kb.B)
	filterLoopsCount := encoding.UnmarshalInt64(kb.B[8:])
	timestamp := encoding.UnmarshalUint64(kb.B[16:])
	return loopsCount, filterLoopsCount, timestamp
}

func (is *indexSearch) storeLoopsCountForDateFilter(date uint64, tf *tagFilter, loopsCount, filterLoopsCount int64) {
	currentTimestamp := fasttime.UnixTimestamp()
	is.kb.B = appendDateTagFilterCacheKey(is.kb.B[:0], is.db.name, date, tf, is.accountID, is.projectID)
	kb := kbPool.Get()
	kb.B = encoding.MarshalInt64(kb.B[:0], loopsCount)
	kb.B = encoding.MarshalInt64(kb.B, filterLoopsCount)
	kb.B = encoding.MarshalUint64(kb.B, currentTimestamp)
	is.db.loopsPerDateTagFilterCache.Set(is.kb.B, kb.B)
	kbPool.Put(kb)
}

func appendDateTagFilterCacheKey(dst []byte, indexDBName string, date uint64, tf *tagFilter, accountID, projectID uint32) []byte {
	dst = append(dst, indexDBName...)
	dst = encoding.MarshalUint64(dst, date)
	dst = tf.Marshal(dst, accountID, projectID)
	return dst
}

func (is *indexSearch) getMetricIDsForDate(date uint64, maxMetrics int) (*uint64set.Set, error) {
	// Extract all the metricIDs from (date, __name__=value)->metricIDs entries.
	kb := kbPool.Get()
	defer kbPool.Put(kb)
	if date != 0 {
		// Use per-date search
		kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixDateTagToMetricIDs)
		kb.B = encoding.MarshalUint64(kb.B, date)
	} else {
		// Use global search
		kb.B = is.marshalCommonPrefix(kb.B[:0], nsPrefixTagToMetricIDs)
	}
	kb.B = marshalTagValue(kb.B, nil)
	var metricIDs uint64set.Set
	if err := is.updateMetricIDsForPrefix(kb.B, &metricIDs, maxMetrics); err != nil {
		return nil, err
	}
	return &metricIDs, nil
}

func (is *indexSearch) updateMetricIDsForPrefix(prefix []byte, metricIDs *uint64set.Set, maxMetrics int) error {
	ts := &is.ts
	mp := &is.mp
	loopsPaceLimiter := 0
	ts.Seek(prefix)
	for ts.NextItem() {
		if loopsPaceLimiter&paceLimiterFastIterationsMask == 0 {
			if err := checkSearchDeadlineAndPace(is.deadline); err != nil {
				return err
			}
		}
		loopsPaceLimiter++
		item := ts.Item
		if !bytes.HasPrefix(item, prefix) {
			return nil
		}
		tail := item[len(prefix):]
		n := bytes.IndexByte(tail, tagSeparatorChar)
		if n < 0 {
			return fmt.Errorf("invalid tag->metricIDs line %q: cannot find tagSeparatorChar %d", item, tagSeparatorChar)
		}
		tail = tail[n+1:]
		if err := mp.InitOnlyTail(item, tail); err != nil {
			return err
		}
		mp.ParseMetricIDs()
		metricIDs.AddMulti(mp.MetricIDs)
		if metricIDs.Len() >= maxMetrics {
			return nil
		}
	}
	if err := ts.Error(); err != nil {
		return fmt.Errorf("error when searching for all metricIDs by prefix %q: %w", prefix, err)
	}
	return nil
}

// The estimated number of index scan loops a single loop in updateMetricIDsByMetricNameMatch takes.
const loopsCountPerMetricNameMatch = 150

var kbPool bytesutil.ByteBufferPool

// Returns local unique MetricID.
func generateUniqueMetricID() uint64 {
	// It is expected that metricIDs returned from this function must be dense.
	// If they will be sparse, then this may hurt metric_ids intersection
	// performance with uint64set.Set.
	return atomic.AddUint64(&nextUniqueMetricID, 1)
}

// This number mustn't go backwards on restarts, otherwise metricID
// collisions are possible. So don't change time on the server
// between VictoriaMetrics restarts.
var nextUniqueMetricID = uint64(time.Now().UnixNano())

func marshalCommonPrefix(dst []byte, nsPrefix byte, accountID, projectID uint32) []byte {
	dst = append(dst, nsPrefix)
	dst = encoding.MarshalUint32(dst, accountID)
	dst = encoding.MarshalUint32(dst, projectID)
	return dst
}

func (is *indexSearch) marshalCommonPrefix(dst []byte, nsPrefix byte) []byte {
	return marshalCommonPrefix(dst, nsPrefix, is.accountID, is.projectID)
}

func unmarshalCommonPrefix(src []byte) ([]byte, byte, uint32, uint32, error) {
	if len(src) < commonPrefixLen {
		return nil, 0, 0, 0, fmt.Errorf("cannot unmarshal common prefix from %d bytes; need at least %d bytes; data=%X", len(src), commonPrefixLen, src)
	}
	prefix := src[0]
	accountID := encoding.UnmarshalUint32(src[1:])
	projectID := encoding.UnmarshalUint32(src[5:])
	return src[commonPrefixLen:], prefix, accountID, projectID, nil
}

// 1 byte for prefix, 4 bytes for accountID, 4 bytes for projectID
const commonPrefixLen = 9

type tagToMetricIDsRowParser struct {
	// NSPrefix contains the first byte parsed from the row after Init call.
	// This is either nsPrefixTagToMetricIDs or nsPrefixDateTagToMetricIDs.
	NSPrefix byte

	// AccountID contains parsed value after Init call
	AccountID uint32

	// ProjectID contains parsed value after Init call
	ProjectID uint32

	// Date contains parsed date for nsPrefixDateTagToMetricIDs rows after Init call
	Date uint64

	// MetricIDs contains parsed MetricIDs after ParseMetricIDs call
	MetricIDs []uint64

	// Tag contains parsed tag after Init call
	Tag Tag

	// tail contains the remaining unparsed metricIDs
	tail []byte
}

func (mp *tagToMetricIDsRowParser) Reset() {
	mp.NSPrefix = 0
	mp.AccountID = 0
	mp.ProjectID = 0
	mp.Date = 0
	mp.MetricIDs = mp.MetricIDs[:0]
	mp.Tag.Reset()
	mp.tail = nil
}

// Init initializes mp from b, which should contain encoded tag->metricIDs row.
//
// b cannot be re-used until Reset call.
func (mp *tagToMetricIDsRowParser) Init(b []byte, nsPrefixExpected byte) error {
	tail, nsPrefix, accountID, projectID, err := unmarshalCommonPrefix(b)
	if err != nil {
		return fmt.Errorf("invalid tag->metricIDs row %q: %w", b, err)
	}
	if nsPrefix != nsPrefixExpected {
		return fmt.Errorf("invalid prefix for tag->metricIDs row %q; got %d; want %d", b, nsPrefix, nsPrefixExpected)
	}
	if nsPrefix == nsPrefixDateTagToMetricIDs {
		// unmarshal date.
		if len(tail) < 8 {
			return fmt.Errorf("cannot unmarshal date from (date, tag)->metricIDs row %q from %d bytes; want at least 8 bytes", b, len(tail))
		}
		mp.Date = encoding.UnmarshalUint64(tail)
		tail = tail[8:]
	}
	mp.NSPrefix = nsPrefix
	mp.AccountID = accountID
	mp.ProjectID = projectID
	tail, err = mp.Tag.Unmarshal(tail)
	if err != nil {
		return fmt.Errorf("cannot unmarshal tag from tag->metricIDs row %q: %w", b, err)
	}
	return mp.InitOnlyTail(b, tail)
}

// MarshalPrefix marshals row prefix without tail to dst.
func (mp *tagToMetricIDsRowParser) MarshalPrefix(dst []byte) []byte {
	dst = marshalCommonPrefix(dst, mp.NSPrefix, mp.AccountID, mp.ProjectID)
	if mp.NSPrefix == nsPrefixDateTagToMetricIDs {
		dst = encoding.MarshalUint64(dst, mp.Date)
	}
	dst = mp.Tag.Marshal(dst)
	return dst
}

// InitOnlyTail initializes mp.tail from tail.
//
// b must contain tag->metricIDs row.
// b cannot be re-used until Reset call.
func (mp *tagToMetricIDsRowParser) InitOnlyTail(b, tail []byte) error {
	if len(tail) == 0 {
		return fmt.Errorf("missing metricID in the tag->metricIDs row %q", b)
	}
	if len(tail)%8 != 0 {
		return fmt.Errorf("invalid tail length in the tag->metricIDs row; got %d bytes; must be multiple of 8 bytes", len(tail))
	}
	mp.tail = tail
	return nil
}

// EqualPrefix returns true if prefixes for mp and x are equal.
//
// Prefix contains (tag, accountID, projectID)
func (mp *tagToMetricIDsRowParser) EqualPrefix(x *tagToMetricIDsRowParser) bool {
	if !mp.Tag.Equal(&x.Tag) {
		return false
	}
	return mp.ProjectID == x.ProjectID && mp.AccountID == x.AccountID && mp.Date == x.Date && mp.NSPrefix == x.NSPrefix
}

// MetricIDsLen returns the number of MetricIDs in the mp.tail
func (mp *tagToMetricIDsRowParser) MetricIDsLen() int {
	return len(mp.tail) / 8
}

// ParseMetricIDs parses MetricIDs from mp.tail into mp.MetricIDs.
func (mp *tagToMetricIDsRowParser) ParseMetricIDs() {
	tail := mp.tail
	mp.MetricIDs = mp.MetricIDs[:0]
	n := len(tail) / 8
	if n <= cap(mp.MetricIDs) {
		mp.MetricIDs = mp.MetricIDs[:n]
	} else {
		mp.MetricIDs = append(mp.MetricIDs[:cap(mp.MetricIDs)], make([]uint64, n-cap(mp.MetricIDs))...)
	}
	metricIDs := mp.MetricIDs
	_ = metricIDs[n-1]
	for i := 0; i < n; i++ {
		if len(tail) < 8 {
			logger.Panicf("BUG: tail cannot be smaller than 8 bytes; got %d bytes; tail=%X", len(tail), tail)
			return
		}
		metricID := encoding.UnmarshalUint64(tail)
		metricIDs[i] = metricID
		tail = tail[8:]
	}
}

// IsDeletedTag verifies whether the tag from mp is deleted according to dmis.
//
// dmis must contain deleted MetricIDs.
func (mp *tagToMetricIDsRowParser) IsDeletedTag(dmis *uint64set.Set) bool {
	if dmis.Len() == 0 {
		return false
	}
	mp.ParseMetricIDs()
	for _, metricID := range mp.MetricIDs {
		if !dmis.Has(metricID) {
			return false
		}
	}
	return true
}

func mergeTagToMetricIDsRows(data []byte, items []mergeset.Item) ([]byte, []mergeset.Item) {
	data, items = mergeTagToMetricIDsRowsInternal(data, items, nsPrefixTagToMetricIDs)
	data, items = mergeTagToMetricIDsRowsInternal(data, items, nsPrefixDateTagToMetricIDs)
	return data, items
}

func mergeTagToMetricIDsRowsInternal(data []byte, items []mergeset.Item, nsPrefix byte) ([]byte, []mergeset.Item) {
	// Perform quick checks whether items contain rows starting from nsPrefix
	// based on the fact that items are sorted.
	if len(items) <= 2 {
		// The first and the last row must remain unchanged.
		return data, items
	}
	firstItem := items[0].Bytes(data)
	if len(firstItem) > 0 && firstItem[0] > nsPrefix {
		return data, items
	}
	lastItem := items[len(items)-1].Bytes(data)
	if len(lastItem) > 0 && lastItem[0] < nsPrefix {
		return data, items
	}

	// items contain at least one row starting from nsPrefix. Merge rows with common tag.
	tmm := getTagToMetricIDsRowsMerger()
	tmm.dataCopy = append(tmm.dataCopy[:0], data...)
	tmm.itemsCopy = append(tmm.itemsCopy[:0], items...)
	mp := &tmm.mp
	mpPrev := &tmm.mpPrev
	dstData := data[:0]
	dstItems := items[:0]
	for i, it := range items {
		item := it.Bytes(data)
		if len(item) == 0 || item[0] != nsPrefix || i == 0 || i == len(items)-1 {
			// Write rows not starting with nsPrefix as-is.
			// Additionally write the first and the last row as-is in order to preserve
			// sort order for adjacent blocks.
			dstData, dstItems = tmm.flushPendingMetricIDs(dstData, dstItems, mpPrev)
			dstData = append(dstData, item...)
			dstItems = append(dstItems, mergeset.Item{
				Start: uint32(len(dstData) - len(item)),
				End:   uint32(len(dstData)),
			})
			continue
		}
		if err := mp.Init(item, nsPrefix); err != nil {
			logger.Panicf("FATAL: cannot parse row starting with nsPrefix %d during merge: %s", nsPrefix, err)
		}
		if mp.MetricIDsLen() >= maxMetricIDsPerRow {
			dstData, dstItems = tmm.flushPendingMetricIDs(dstData, dstItems, mpPrev)
			dstData = append(dstData, item...)
			dstItems = append(dstItems, mergeset.Item{
				Start: uint32(len(dstData) - len(item)),
				End:   uint32(len(dstData)),
			})
			continue
		}
		if !mp.EqualPrefix(mpPrev) {
			dstData, dstItems = tmm.flushPendingMetricIDs(dstData, dstItems, mpPrev)
		}
		mp.ParseMetricIDs()
		tmm.pendingMetricIDs = append(tmm.pendingMetricIDs, mp.MetricIDs...)
		mpPrev, mp = mp, mpPrev
		if len(tmm.pendingMetricIDs) >= maxMetricIDsPerRow {
			dstData, dstItems = tmm.flushPendingMetricIDs(dstData, dstItems, mpPrev)
		}
	}
	if len(tmm.pendingMetricIDs) > 0 {
		logger.Panicf("BUG: tmm.pendingMetricIDs must be empty at this point; got %d items: %d", len(tmm.pendingMetricIDs), tmm.pendingMetricIDs)
	}
	if !checkItemsSorted(dstData, dstItems) {
		// Items could become unsorted if initial items contain duplicate metricIDs:
		//
		//   item1: 1, 1, 5
		//   item2: 1, 4
		//
		// Items could become the following after the merge:
		//
		//   item1: 1, 5
		//   item2: 1, 4
		//
		// i.e. item1 > item2
		//
		// Leave the original items unmerged, so they can be merged next time.
		// This case should be quite rare - if multiple data points are simultaneously inserted
		// into the same new time series from multiple concurrent goroutines.
		atomic.AddUint64(&indexBlocksWithMetricIDsIncorrectOrder, 1)
		dstData = append(dstData[:0], tmm.dataCopy...)
		dstItems = append(dstItems[:0], tmm.itemsCopy...)
		if !checkItemsSorted(dstData, dstItems) {
			logger.Panicf("BUG: the original items weren't sorted; items=%q", dstItems)
		}
	}
	putTagToMetricIDsRowsMerger(tmm)
	atomic.AddUint64(&indexBlocksWithMetricIDsProcessed, 1)
	return dstData, dstItems
}

var indexBlocksWithMetricIDsIncorrectOrder uint64
var indexBlocksWithMetricIDsProcessed uint64

func checkItemsSorted(data []byte, items []mergeset.Item) bool {
	if len(items) == 0 {
		return true
	}
	prevItem := items[0].String(data)
	for _, it := range items[1:] {
		currItem := it.String(data)
		if prevItem > currItem {
			return false
		}
		prevItem = currItem
	}
	return true
}

// maxMetricIDsPerRow limits the number of metricIDs in tag->metricIDs row.
//
// This reduces overhead on index and metaindex in lib/mergeset.
const maxMetricIDsPerRow = 64

type uint64Sorter []uint64

func (s uint64Sorter) Len() int { return len(s) }
func (s uint64Sorter) Less(i, j int) bool {
	return s[i] < s[j]
}
func (s uint64Sorter) Swap(i, j int) {
	s[i], s[j] = s[j], s[i]
}

type tagToMetricIDsRowsMerger struct {
	pendingMetricIDs uint64Sorter
	mp               tagToMetricIDsRowParser
	mpPrev           tagToMetricIDsRowParser

	itemsCopy []mergeset.Item
	dataCopy  []byte
}

func (tmm *tagToMetricIDsRowsMerger) Reset() {
	tmm.pendingMetricIDs = tmm.pendingMetricIDs[:0]
	tmm.mp.Reset()
	tmm.mpPrev.Reset()

	tmm.itemsCopy = tmm.itemsCopy[:0]
	tmm.dataCopy = tmm.dataCopy[:0]
}

func (tmm *tagToMetricIDsRowsMerger) flushPendingMetricIDs(dstData []byte, dstItems []mergeset.Item, mp *tagToMetricIDsRowParser) ([]byte, []mergeset.Item) {
	if len(tmm.pendingMetricIDs) == 0 {
		// Nothing to flush
		return dstData, dstItems
	}
	// Use sort.Sort instead of sort.Slice in order to reduce memory allocations.
	sort.Sort(&tmm.pendingMetricIDs)
	tmm.pendingMetricIDs = removeDuplicateMetricIDs(tmm.pendingMetricIDs)

	// Marshal pendingMetricIDs
	dstDataLen := len(dstData)
	dstData = mp.MarshalPrefix(dstData)
	for _, metricID := range tmm.pendingMetricIDs {
		dstData = encoding.MarshalUint64(dstData, metricID)
	}
	dstItems = append(dstItems, mergeset.Item{
		Start: uint32(dstDataLen),
		End:   uint32(len(dstData)),
	})
	tmm.pendingMetricIDs = tmm.pendingMetricIDs[:0]
	return dstData, dstItems
}

func removeDuplicateMetricIDs(sortedMetricIDs []uint64) []uint64 {
	if len(sortedMetricIDs) < 2 {
		return sortedMetricIDs
	}
	prevMetricID := sortedMetricIDs[0]
	hasDuplicates := false
	for _, metricID := range sortedMetricIDs[1:] {
		if prevMetricID == metricID {
			hasDuplicates = true
			break
		}
		prevMetricID = metricID
	}
	if !hasDuplicates {
		return sortedMetricIDs
	}
	dstMetricIDs := sortedMetricIDs[:1]
	prevMetricID = sortedMetricIDs[0]
	for _, metricID := range sortedMetricIDs[1:] {
		if prevMetricID == metricID {
			continue
		}
		dstMetricIDs = append(dstMetricIDs, metricID)
		prevMetricID = metricID
	}
	return dstMetricIDs
}

func getTagToMetricIDsRowsMerger() *tagToMetricIDsRowsMerger {
	v := tmmPool.Get()
	if v == nil {
		return &tagToMetricIDsRowsMerger{}
	}
	return v.(*tagToMetricIDsRowsMerger)
}

func putTagToMetricIDsRowsMerger(tmm *tagToMetricIDsRowsMerger) {
	tmm.Reset()
	tmmPool.Put(tmm)
}

var tmmPool sync.Pool