persistedsqlstats: add AOST clause to statement statistics size check

This change as an AOST clause to the statement statistics size check.
This helps mitigate contention on the `system.statement_statistics`
table which has caused the sql stats compaction job to fail.

Release note (bug fix): this change reduces contention on the
`system.statement_statistics` table which has caused the SQL statistics
compaction job to fail

pkg/sql/sqlstats/persistedsqlstats/flush.go

@@ -72,7 +72,12 @@ func (s *PersistedSQLStats) Flush(ctx context.Context) {
 
 	aggregatedTs := s.ComputeAggregatedTs()
 
-	if s.stmtsLimitSizeReached(ctx) || s.txnsLimitSizeReached(ctx) {
+	// We only check the statement count as there should always be at least as many statements as transactions.
+	limitReached, err := s.StmtsLimitSizeReached(ctx)
+	if err != nil {
+		log.Errorf(ctx, "encountered an error at flush, checking for statement statistics size limit: %v", err)
+	}
+	if limitReached {
 		log.Infof(ctx, "unable to flush fingerprints because table limit was reached.")
 	} else {
 		var wg sync.WaitGroup
@@ -92,7 +97,7 @@ func (s *PersistedSQLStats) Flush(ctx context.Context) {
 	}
 }
 
-func (s *PersistedSQLStats) stmtsLimitSizeReached(ctx context.Context) bool {
+func (s *PersistedSQLStats) StmtsLimitSizeReached(ctx context.Context) (bool, error) {
 	maxPersistedRows := float64(SQLStatsMaxPersistedRows.Get(&s.SQLStats.GetClusterSettings().SV))
 
 	readStmt := `
@@ -101,7 +106,7 @@ SELECT
 FROM
     system.statement_statistics
 `
-
+	readStmt += s.cfg.Knobs.GetAOSTClause()
 	row, err := s.cfg.DB.Executor().QueryRowEx(
 		ctx,
 		"fetch-stmt-count",
@@ -111,35 +116,10 @@ FROM
 	)
 
 	if err != nil {
-		return false
-	}
-	actualSize := float64(tree.MustBeDInt(row[0]))
-	return actualSize > (maxPersistedRows * 1.5)
-}
-
-func (s *PersistedSQLStats) txnsLimitSizeReached(ctx context.Context) bool {
-	maxPersistedRows := float64(SQLStatsMaxPersistedRows.Get(&s.SQLStats.GetClusterSettings().SV))
-
-	readStmt := `
-SELECT
-    count(*)
-FROM
-    system.transaction_statistics
-`
-
-	row, err := s.cfg.DB.Executor().QueryRowEx(
-		ctx,
-		"fetch-txn-count",
-		nil,
-		sessiondata.NodeUserSessionDataOverride,
-		readStmt,
-	)
-
-	if err != nil {
-		return false
+		return false, err
 	}
 	actualSize := float64(tree.MustBeDInt(row[0]))
-	return actualSize > (maxPersistedRows * 1.5)
+	return actualSize > (maxPersistedRows * 1.5), nil
 }
 
 func (s *PersistedSQLStats) flushStmtStats(ctx context.Context, aggregatedTs time.Time) {

pkg/sql/sqlstats/persistedsqlstats/flush_test.go

@@ -527,6 +527,64 @@ func TestSQLStatsPersistedLimitReached(t *testing.T) {
 	require.Equal(t, txnStatsCountFlush3, txnStatsCountFlush2)
 }
 
+func TestSQLStatsReadLimitSizeOnLockedTable(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+	defer log.Scope(t).Close(t)
+
+	ctx := context.Background()
+	s, conn, _ := serverutils.StartServer(t, base.TestServerArgs{})
+	defer s.Stopper().Stop(context.Background())
+	sqlConn := sqlutils.MakeSQLRunner(conn)
+	sqlConn.Exec(t, `INSERT INTO system.users VALUES ('node', NULL, true, 3); GRANT node TO root`)
+	waitForFollowerReadTimestamp(t, sqlConn)
+	pss := s.SQLServer().(*sql.Server).GetSQLStatsProvider().(*persistedsqlstats.PersistedSQLStats)
+
+	const minNumExpectedStmts = int64(3)
+	// Maximum number of persisted rows less than minNumExpectedStmts/1.5
+	const maxNumPersistedRows = 1
+	sqlConn.Exec(t, "SELECT 1")
+	sqlConn.Exec(t, "SELECT 1, 2")
+	sqlConn.Exec(t, "SELECT 1, 2, 3")
+
+	pss.Flush(ctx)
+	stmtStatsCountFlush, _ := countStats(t, sqlConn)
+
+	// Ensure we have some rows in system.statement_statistics
+	require.GreaterOrEqual(t, stmtStatsCountFlush, minNumExpectedStmts)
+
+	// Set sql.stats.persisted_rows.max
+	sqlConn.Exec(t, fmt.Sprintf("SET CLUSTER SETTING sql.stats.persisted_rows.max=%d", maxNumPersistedRows))
+	testutils.SucceedsSoon(t, func() error {
+		var appliedSetting int
+		row := sqlConn.QueryRow(t, "SHOW CLUSTER SETTING sql.stats.persisted_rows.max")
+		row.Scan(&appliedSetting)
+		if appliedSetting != maxNumPersistedRows {
+			return errors.Newf("waiting for sql.stats.persisted_rows.max to be applied")
+		}
+		return nil
+	})
+
+	// Begin a transaction.
+	sqlConn.Exec(t, "BEGIN")
+	// Lock the table. Create a state of contention.
+	sqlConn.Exec(t, "SELECT * FROM system.statement_statistics FOR UPDATE")
+
+	// Ensure that we can read from the table despite it being locked, due to the follower read (AOST).
+	// Expect that the number of statements in the table exceeds sql.stats.persisted_rows.max * 1.5
+	// (meaning that the limit will be reached) and no error. We need SucceedsSoon here for the follower
+	// read timestamp to catch up enough for this state to be reached.
+	testutils.SucceedsSoon(t, func() error {
+		limitReached, err := pss.StmtsLimitSizeReached(ctx)
+		if limitReached != true {
+			return errors.New("waiting for limit reached to be true")
+		}
+		return err
+	})
+
+	// Close the transaction.
+	sqlConn.Exec(t, "COMMIT")
+}
+
 func TestSQLStatsPlanSampling(t *testing.T) {
 	defer leaktest.AfterTest(t)()
 	defer log.Scope(t).Close(t)
@@ -824,3 +882,17 @@ WHERE
 	require.Equal(t, nodeID, gatewayNodeID, "Gateway NodeID")
 	require.Equal(t, "[1]", allNodesIds, "All NodeIDs from statistics")
 }
+
+func waitForFollowerReadTimestamp(t *testing.T, sqlConn *sqlutils.SQLRunner) {
+	var hlcTimestamp time.Time
+	sqlConn.QueryRow(t, `SELECT hlc_to_timestamp(cluster_logical_timestamp())`).Scan(&hlcTimestamp)
+
+	testutils.SucceedsSoon(t, func() error {
+		var followerReadTimestamp time.Time
+		sqlConn.QueryRow(t, `SELECT follower_read_timestamp()`).Scan(&followerReadTimestamp)
+		if followerReadTimestamp.Before(hlcTimestamp) {
+			return errors.New("waiting for follower_read_timestamp to be passed hlc timestamp")
+		}
+		return nil
+	})
+}