From 74fe2f4cdc73a7dc276f1526154965c3c926cfbe Mon Sep 17 00:00:00 2001
From: Aliaksandr Valialkin <valyala@victoriametrics.com>
Date: Tue, 4 Jun 2024 15:21:08 +0200
Subject: [PATCH] wip

---
 app/vlselect/logsql/logsql.go | 28 +++++++++++++++-------------
 1 file changed, 15 insertions(+), 13 deletions(-)

diff --git a/app/vlselect/logsql/logsql.go b/app/vlselect/logsql/logsql.go
index f74c63735..fcc16f24a 100644
--- a/app/vlselect/logsql/logsql.go
+++ b/app/vlselect/logsql/logsql.go
@@ -355,19 +355,20 @@ type row struct {
 }
 
 func getLastNQueryResults(ctx context.Context, tenantIDs []logstorage.TenantID, q *logstorage.Query, limit int) ([]row, error) {
-	q.AddPipeLimit(uint64(limit + 1))
+	limitUpper := 2*limit
+	q.AddPipeLimit(uint64(limitUpper))
 	q.Optimize()
-	rows, err := getQueryResultsWithLimit(ctx, tenantIDs, q, limit+1)
+	rows, err := getQueryResultsWithLimit(ctx, tenantIDs, q, limitUpper)
 	if err != nil {
 		return nil, err
 	}
-	if len(rows) <= limit {
-		// Fast path - the requested time range contains up to limit rows.
-		sortRowsByTime(rows)
+	if len(rows) < limitUpper {
+		// Fast path - the requested time range contains up to limitUpper rows.
+		rows = getLastNRows(rows, limit)
 		return rows, nil
 	}
 
-	// Slow path - search for the time range with the requested limit rows.
+	// Slow path - search for the time range containing up to limitUpper rows.
 	start, end := q.GetFilterTimeRange()
 	d := end/2 - start/2
 	start += d
@@ -376,16 +377,13 @@ func getLastNQueryResults(ctx context.Context, tenantIDs []logstorage.TenantID,
 	for {
 		q = qOrig.Clone()
 		q.AddTimeFilter(start, end)
-		rows, err := getQueryResultsWithLimit(ctx, tenantIDs, q, limit+1)
+		rows, err := getQueryResultsWithLimit(ctx, tenantIDs, q, limitUpper)
 		if err != nil {
 			return nil, err
 		}
 
-		if len(rows) == limit || len(rows) > limit && d < 10e6 || d == 0 {
-			sortRowsByTime(rows)
-			if len(rows) > limit {
-				rows = rows[len(rows)-limit:]
-			}
+		if len(rows) >= limit && len(rows) < limitUpper || d == 0 {
+			rows = getLastNRows(rows, limit)
 			return rows, nil
 		}
 
@@ -399,10 +397,14 @@ func getLastNQueryResults(ctx context.Context, tenantIDs []logstorage.TenantID,
 	}
 }
 
-func sortRowsByTime(rows []row) {
+func getLastNRows(rows []row, limit int) []row {
 	sort.Slice(rows, func(i, j int) bool {
 		return rows[i].timestamp < rows[j].timestamp
 	})
+	if len(rows) > limit {
+		rows = rows[len(rows)-limit:]
+	}
+	return rows
 }
 
 func getQueryResultsWithLimit(ctx context.Context, tenantIDs []logstorage.TenantID, q *logstorage.Query, limit int) ([]row, error) {