stats: fix panic when fast analyze on empty table (pingcap#13284) (pi…

…ngcap#13344)

executor/analyze.go

@@ -1089,7 +1089,9 @@ func (e *AnalyzeFastExec) runTasks() ([]*statistics.Histogram, []*statistics.CMS
 		sort.Slice(collector.Samples, func(i, j int) bool { return collector.Samples[i].RowID < collector.Samples[j].RowID })
 		collector.CalcTotalSize()
 		// Scale the total column size.
-		collector.TotalSize *= rowCount / int64(len(collector.Samples))
+		if len(collector.Samples) > 0 {
+			collector.TotalSize *= rowCount / int64(len(collector.Samples))
+		}
 		if i < hasPKInfo {
 			hists[i], cms[i], err = e.buildColumnStats(e.pkInfo.ID, e.collectors[i], &e.pkInfo.FieldType, rowCount)
 		} else if i < hasPKInfo+len(e.colsInfo) {

executor/analyze_test.go

@@ -247,6 +247,8 @@ func (s *testSuite1) TestFastAnalyze(c *C) {
 	tk.MustExec("create table t(a int primary key, b int, c char(10), index index_b(b))")
 	tk.MustExec("set @@session.tidb_enable_fast_analyze=1")
 	tk.MustExec("set @@session.tidb_build_stats_concurrency=1")
+	// Should not panic.
+	tk.MustExec("analyze table t")
 	tblInfo, err := dom.InfoSchema().TableByName(model.NewCIStr("test"), model.NewCIStr("t"))
 	c.Assert(err, IsNil)
 	tid := tblInfo.Meta().ID

statistics/cmsketch.go

@@ -111,6 +111,9 @@ func newTopNHelper(sample [][]byte, numTop uint32) *topNHelper {
 
 // NewCMSketchWithTopN returns a new CM sketch with TopN elements, the estimate NDV and the scale ratio.
 func NewCMSketchWithTopN(d, w int32, sample [][]byte, numTop uint32, rowCount uint64) (*CMSketch, uint64, uint64) {
+	if rowCount == 0 || len(sample) == 0 {
+		return nil, 0, 0
+	}
 	helper := newTopNHelper(sample, numTop)
 	// rowCount is not a accurate value when fast analyzing
 	// In some cases, if user triggers fast analyze when rowCount is close to sampleSize, unexpected bahavior might happen.