*: show cmd to check if all needed histograms are loaded

executor/show.go

@@ -196,6 +196,9 @@ func (e *ShowExec) fetchAll(ctx context.Context) error {
 	case ast.ShowStatsHealthy:
 		e.fetchShowStatsHealthy()
 		return nil
+	case ast.ShowHistogramsInFlight:
+		e.fetchShowHistogramsInFlight()
+		return nil
 	case ast.ShowColumnStatsUsage:
 		return e.fetchShowColumnStatsUsage()
 	case ast.ShowPlugins:

executor/show_stats.go

@@ -443,6 +443,10 @@ func (e *ShowExec) appendTableForStatsHealthy(dbName, tblName, partitionName str
 	})
 }
 
+func (e *ShowExec) fetchShowHistogramsInFlight() {
+	e.appendRow([]interface{}{statistics.HistogramNeededColumns.Length()})
+}
+
 func (e *ShowExec) fetchShowAnalyzeStatus() {
 	rows := dataForAnalyzeStatusHelper(e.baseExecutor.ctx)
 	for _, row := range rows {

executor/show_stats_test.go

@@ -350,3 +350,16 @@ func TestShowColumnStatsUsage(t *testing.T) {
 	require.Equal(t, rows[0], []interface{}{"test", "t2", "global", t1.Meta().Columns[0].Name.O, "2021-10-20 09:00:00", "<nil>"})
 	require.Equal(t, rows[1], []interface{}{"test", "t2", p0.Name.O, t1.Meta().Columns[0].Name.O, "2021-10-20 09:00:00", "<nil>"})
 }
+
+func TestShowHistogramsInFlight(t *testing.T) {
+	t.Parallel()
+	store, clean := testkit.CreateMockStore(t)
+	defer clean()
+
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	result := tk.MustQuery("show histograms_in_flight")
+	rows := result.Rows()
+	require.Equal(t, len(rows), 1)
+	require.Equal(t, rows[0][0], "0")
+}

parser/ast/dml.go

@@ -2580,6 +2580,7 @@ const (
 	ShowStatsTopN
 	ShowStatsBuckets
 	ShowStatsHealthy
+	ShowHistogramsInFlight
 	ShowColumnStatsUsage
 	ShowPlugins
 	ShowProfile
@@ -2768,6 +2769,11 @@ func (n *ShowStmt) Restore(ctx *format.RestoreCtx) error {
 		if err := restoreShowLikeOrWhereOpt(); err != nil {
 			return err
 		}
+	case ShowHistogramsInFlight:
+		ctx.WriteKeyWord("HISTOGRAMS_IN_FLIGHT")
+		if err := restoreShowLikeOrWhereOpt(); err != nil {
+			return err
+		}
 	case ShowColumnStatsUsage:
 		ctx.WriteKeyWord("COLUMN_STATS_USAGE")
 		if err := restoreShowLikeOrWhereOpt(); err != nil {

parser/misc.go

@@ -670,6 +670,7 @@ var tokenMap = map[string]int{
 	"STATS_HISTOGRAMS":         statsHistograms,
 	"STATS_TOPN":               statsTopN,
 	"STATS_META":               statsMeta,
+	"HISTOGRAMS_IN_FLIGHT":     histogramsInFlight,
 	"STATS_PERSISTENT":         statsPersistent,
 	"STATS_SAMPLE_PAGES":       statsSamplePages,
 	"STATS":                    stats,