lib/logstorage: avoid reading columnsHeader data when field_values pipe is applied directly to log filters

This improves performance of `field_values` pipe when it is applied to large number of data blocks.
This also improves performance of /select/logsql/field_values HTTP API.

docs/VictoriaLogs/CHANGELOG.md

@@ -16,6 +16,7 @@ according to [these docs](https://docs.victoriametrics.com/victorialogs/quicksta
 ## tip
 
 * FEATURE: added ability to receive systemd (journald) logs over network. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4618).
+* FEATURE: improve performance for [`field_values` pipe](https://docs.victoriametrics.com/victorialogs/logsql/#field_values-pipe) when it is applied directly to [log filter](https://docs.victoriametrics.com/victorialogs/logsql/#filters).
 
 * BUGFIX: [web UI](https://docs.victoriametrics.com/victorialogs/querying/#web-ui): fix various glitches with updating query responses. The issue was introduced in [v0.36.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v0.36.0-victorialogs). See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7279).
 

lib/logstorage/block_search.go

@@ -337,6 +337,14 @@ func (bs *blockSearch) getColumnNameID(name string) (uint64, bool) {
 	return id, ok
 }
 
+func (bs *blockSearch) getColumnNameByID(columnNameID uint64) string {
+	columnNames := bs.bsw.p.columnNames
+	if columnNameID >= uint64(len(columnNames)) {
+		logger.Panicf("FATAL: %s: too big columnNameID=%d; it must be smaller than %d", bs.bsw.p.path, columnNameID, len(columnNames))
+	}
+	return columnNames[columnNameID]
+}
+
 func (bs *blockSearch) getColumnsHeaderIndex() *columnsHeaderIndex {
 	if bs.partFormatVersion() < 1 {
 		logger.Panicf("BUG: getColumnsHeaderIndex() can be called only for part encoding v1+, while it has been called for v%d", bs.partFormatVersion())

lib/logstorage/parser_test.go

@@ -1885,12 +1885,12 @@ func TestQueryGetNeededColumns(t *testing.T) {
 	f(`* | fields x,f1 | filter foo f1:bar | rm f2`, `f1,x`, ``)
 	f(`* | rm x,f1 | filter foo f1:bar`, `*`, `f1,x`)
 
-	f(`* | field_names as foo`, `*`, `_time`)
-	f(`* | field_names foo | fields bar`, `*`, `_time`)
-	f(`* | field_names foo | fields foo`, `*`, `_time`)
-	f(`* | field_names foo | rm foo`, `*`, `_time`)
-	f(`* | field_names foo | rm bar`, `*`, `_time`)
-	f(`* | field_names foo | rm _time`, `*`, `_time`)
+	f(`* | field_names as foo`, ``, ``)
+	f(`* | field_names foo | fields bar`, ``, ``)
+	f(`* | field_names foo | fields foo`, ``, ``)
+	f(`* | field_names foo | rm foo`, ``, ``)
+	f(`* | field_names foo | rm bar`, ``, ``)
+	f(`* | field_names foo | rm _time`, ``, ``)
 	f(`* | fields x,y | field_names as bar | fields baz`, `x,y`, ``)
 	f(`* | rm x,y | field_names as bar | fields baz`, `*`, `x,y`)
 
@@ -2012,7 +2012,7 @@ func TestQueryGetNeededColumns(t *testing.T) {
 	f(`* | extract if (q:w p:a) "<f1>bar" from x | count() r1`, `p,q`, ``)
 	f(`* | extract_regexp "(?P<f1>.*)bar" from x | count() r1`, ``, ``)
 	f(`* | extract_regexp if (q:w p:a) "(?P<f1>.*)bar" from x | count() r1`, `p,q`, ``)
-	f(`* | field_names | count() r1`, `*`, `_time`)
+	f(`* | field_names | count() r1`, ``, ``)
 	f(`* | limit 10 | field_names as abc | count() r1`, `*`, ``)
 	f(`* | blocks_count | count() r1`, ``, ``)
 	f(`* | limit 10 | blocks_count as abc | count() r1`, ``, ``)

lib/logstorage/pipe_field_names.go

@@ -14,9 +14,7 @@ type pipeFieldNames struct {
 	// By default results are written into 'name' column.
 	resultName string
 
-	// isFirstPipe is set to true if '| field_names' pipe is the first in the query.
-	//
-	// This allows skipping loading of _time column.
+	// if isFirstPipe is set, then there is no need in loading columnsHeader in writeBlock().
 	isFirstPipe bool
 }
 
@@ -33,12 +31,12 @@ func (pf *pipeFieldNames) canLiveTail() bool {
 }
 
 func (pf *pipeFieldNames) updateNeededFields(neededFields, unneededFields fieldsSet) {
-	neededFields.add("*")
-	unneededFields.reset()
-
 	if pf.isFirstPipe {
-		unneededFields.add("_time")
+		neededFields.reset()
+	} else {
+		neededFields.add("*")
 	}
+	unneededFields.reset()
 }
 
 func (pf *pipeFieldNames) optimize() {
@@ -98,25 +96,48 @@ func (pfp *pipeFieldNamesProcessor) writeBlock(workerID uint, br *blockResult) {
 		return
 	}
 
+	// Assume that the column is set for all the rows in the block.
+	// This is much faster than reading all the column values and counting non-empty rows.
+	hits := uint64(br.rowsLen)
+
 	shard := &pfp.shards[workerID]
-	m := shard.getM()
-
-	cs := br.getColumns()
-	for _, c := range cs {
-		pHits := m[c.name]
-		if pHits == nil {
-			nameCopy := strings.Clone(c.name)
-			hits := uint64(0)
-			pHits = &hits
-			m[nameCopy] = pHits
+	if !pfp.pf.isFirstPipe || br.bs == nil || br.bs.partFormatVersion() < 1 {
+		cs := br.getColumns()
+		for _, c := range cs {
+			shard.updateColumnHits(c.name, hits)
 		}
-
-		// Assume that the column is set for all the rows in the block.
-		// This is much faster than reading all the column values and counting non-empty rows.
-		*pHits += uint64(br.rowsLen)
+	} else {
+		cshIndex := br.bs.getColumnsHeaderIndex()
+		shard.updateHits(cshIndex.columnHeadersRefs, br, hits)
+		shard.updateHits(cshIndex.constColumnsRefs, br, hits)
+		shard.updateColumnHits("_time", hits)
+		shard.updateColumnHits("_stream", hits)
+		shard.updateColumnHits("_stream_id", hits)
 	}
 }
 
+func (shard *pipeFieldNamesProcessorShard) updateHits(refs []columnHeaderRef, br *blockResult, hits uint64) {
+	for _, cr := range refs {
+		columnName := br.bs.getColumnNameByID(cr.columnNameID)
+		shard.updateColumnHits(columnName, hits)
+	}
+}
+
+func (shard *pipeFieldNamesProcessorShard) updateColumnHits(columnName string, hits uint64) {
+	if columnName == "" {
+		columnName = "_msg"
+	}
+	m := shard.getM()
+	pHits := m[columnName]
+	if pHits == nil {
+		nameCopy := strings.Clone(columnName)
+		hits := uint64(0)
+		pHits = &hits
+		m[nameCopy] = pHits
+	}
+	*pHits += hits
+}
+
 func (pfp *pipeFieldNamesProcessor) flush() error {
 	if needStop(pfp.stopCh) {
 		return nil
@@ -136,14 +157,6 @@ func (pfp *pipeFieldNamesProcessor) flush() error {
 			}
 		}
 	}
-	if pfp.pf.isFirstPipe {
-		pHits := m["_stream"]
-		if pHits == nil {
-			hits := uint64(0)
-			pHits = &hits
-		}
-		m["_time"] = pHits
-	}
 
 	// write result
 	wctx := &pipeFieldNamesWriteContext{