stats: fix bug when build histograms for null json (pingcap#5545)

statistics/builder.go

@@ -164,8 +164,13 @@ func BuildColumn(ctx context.Context, numBuckets, id int64, collector *SampleCol
 	}
 	bucketIdx := 0
 	var lastCount int64
-	hg.Buckets[0].LowerBound = samples[0]
-	for i := int64(0); i < int64(len(samples)); i++ {
+	hg.Buckets[0] = Bucket{
+		LowerBound: samples[0],
+		UpperBound: samples[0],
+		Count:      int64(sampleFactor),
+		Repeats:    int64(ndvFactor),
+	}
+	for i := int64(1); i < int64(len(samples)); i++ {
 		cmp, err := hg.Buckets[bucketIdx].UpperBound.CompareDatum(sc, &samples[i])
 		if err != nil {
 			return nil, errors.Trace(err)

statistics/statistics_test.go

@@ -28,6 +28,7 @@ import (
 	"github.com/pingcap/tidb/util/codec"
 	"github.com/pingcap/tidb/util/mock"
 	"github.com/pingcap/tidb/util/types"
+	"github.com/pingcap/tidb/util/types/json"
 )
 
 func TestT(t *testing.T) {
@@ -43,11 +44,6 @@ type testStatisticsSuite struct {
 	pk      ast.RecordSet
 }
 
-type dataTable struct {
-	count   int64
-	samples []types.Datum
-}
-
 type recordSet struct {
 	data   []types.Datum
 	count  int64
@@ -271,6 +267,19 @@ func (s *testStatisticsSuite) TestBuild(c *C) {
 	count, err = col.lessRowCount(sc, types.NewIntDatum(99999))
 	c.Check(err, IsNil)
 	c.Check(int(count), Equals, 99999)
+
+	datum := types.Datum{}
+	datum.SetMysqlJSON(json.JSON{TypeCode: json.TypeCodeLiteral})
+	collector = &SampleCollector{
+		Count:     1,
+		NullCount: 0,
+		Samples:   []types.Datum{datum},
+		Sketch:    sketch,
+	}
+	col, err = BuildColumn(ctx, bucketCount, 2, collector)
+	c.Assert(err, IsNil)
+	c.Assert(len(col.Buckets), Equals, 1)
+	c.Assert(col.Buckets[0].LowerBound, DeepEquals, col.Buckets[0].UpperBound)
 }
 
 func (s *testStatisticsSuite) TestHistogramProtoConversion(c *C) {