statistics: skip reading mysql.stats_histograms if cached stats is up…

…-to-date (#24175) (#24317)

statistics/handle/handle.go

@@ -266,6 +266,9 @@ func (h *Handle) Update(is infoschema.InfoSchema) error {
 			continue
 		}
 		tableInfo := table.Meta()
+		if oldTbl, ok := oldCache.tables[physicalID]; ok && oldTbl.Version >= version && tableInfo.UpdateTS == oldTbl.TblInfoUpdateTS {
+			continue
+		}
 		tbl, err := h.TableStatsFromStorage(tableInfo, physicalID, false, 0)
 		// Error is not nil may mean that there are some ddl changes on this table, we will not update it.
 		if err != nil {
@@ -280,6 +283,7 @@ func (h *Handle) Update(is infoschema.InfoSchema) error {
 		tbl.Count = count
 		tbl.ModifyCount = modifyCount
 		tbl.Name = getFullTableName(is, tableInfo)
+		tbl.TblInfoUpdateTS = tableInfo.UpdateTS
 		tables = append(tables, tbl)
 	}
 	h.updateStatsCache(oldCache.update(tables, deletedTableIDs, lastVersion))

statistics/handle/handle_test.go

@@ -2133,3 +2133,24 @@ func (s *testStatsSuite) TestDuplicateFMSketch(c *C) {
 	c.Assert(s.do.StatsHandle().GCStats(s.do.InfoSchema(), time.Duration(0)), IsNil)
 	tk.MustQuery("select count(*) from mysql.stats_fm_sketch").Check(testkit.Rows("2"))
 }
+
+func (s *testStatsSuite) TestStatsCacheUpdateSkip(c *C) {
+	defer cleanEnv(c, s.store, s.do)
+	testKit := testkit.NewTestKit(c, s.store)
+	do := s.do
+	h := do.StatsHandle()
+	testKit.MustExec("use test")
+	testKit.MustExec("create table t (c1 int, c2 int)")
+	testKit.MustExec("insert into t values(1, 2)")
+	c.Assert(h.DumpStatsDeltaToKV(handle.DumpAll), IsNil)
+	testKit.MustExec("analyze table t")
+	is := do.InfoSchema()
+	tbl, err := is.TableByName(model.NewCIStr("test"), model.NewCIStr("t"))
+	c.Assert(err, IsNil)
+	tableInfo := tbl.Meta()
+	statsTbl1 := h.GetTableStats(tableInfo)
+	c.Assert(statsTbl1.Pseudo, IsFalse)
+	h.Update(is)
+	statsTbl2 := h.GetTableStats(tableInfo)
+	c.Assert(statsTbl1, Equals, statsTbl2)
+}

statistics/table.go

@@ -62,6 +62,12 @@ type Table struct {
 	Version       uint64
 	Name          string
 	ExtendedStats *ExtendedStatsColl
+	// TblInfoUpdateTS is the UpdateTS of the TableInfo used when filling this struct.
+	// It is the schema version of the corresponding table. It is used to skip redundant
+	// loading of stats, i.e, if the cached stats is already update-to-date with mysql.stats_xxx tables,
+	// and the schema of the table does not change, we don't need to load the stats for this
+	// table again.
+	TblInfoUpdateTS uint64
 }
 
 // ExtendedStatsItem is the cached item of a mysql.stats_extended record.
@@ -136,9 +142,10 @@ func (t *Table) Copy() *Table {
 		newHistColl.Indices[id] = idx
 	}
 	nt := &Table{
-		HistColl: newHistColl,
-		Version:  t.Version,
-		Name:     t.Name,
+		HistColl:        newHistColl,
+		Version:         t.Version,
+		Name:            t.Name,
+		TblInfoUpdateTS: t.TblInfoUpdateTS,
 	}
 	if t.ExtendedStats != nil {
 		newExtStatsColl := &ExtendedStatsColl{