statistics: fix old bucket4Merging to merge new data (#49169) (#49187)

close #49168

pkg/statistics/BUILD.bazel

@@ -42,6 +42,7 @@ go_library(
         "//pkg/util/dbterror",
         "//pkg/util/fastrand",
         "//pkg/util/hack",
+        "//pkg/util/intest",
         "//pkg/util/logutil",
         "//pkg/util/mathutil",
         "//pkg/util/memory",

pkg/statistics/histogram.go

@@ -39,6 +39,7 @@ import (
 	"github.com/pingcap/tidb/pkg/util/chunk"
 	"github.com/pingcap/tidb/pkg/util/codec"
 	"github.com/pingcap/tidb/pkg/util/collate"
+	"github.com/pingcap/tidb/pkg/util/intest"
 	"github.com/pingcap/tidb/pkg/util/logutil"
 	"github.com/pingcap/tidb/pkg/util/ranger"
 	"github.com/pingcap/tipb/go-tipb"
@@ -1180,6 +1181,9 @@ func newbucket4MergingForRecycle() *bucket4Merging {
 
 func releasebucket4MergingForRecycle(b *bucket4Merging) {
 	b.disjointNDV = 0
+	b.Repeat = 0
+	b.NDV = 0
+	b.Count = 0
 	bucket4MergingPool.Put(b)
 }
 
@@ -1367,6 +1371,9 @@ func mergePartitionBuckets(sc *stmtctx.StatementContext, buckets []*bucket4Mergi
 	right := buckets[len(buckets)-1].Clone()
 
 	totNDV := int64(0)
+	intest.Assert(res.Count == 0, "Count in the new bucket4Merging should be 0")
+	intest.Assert(res.Repeat == 0, "Repeat in the new bucket4Merging should be 0")
+	intest.Assert(res.NDV == 0, "NDV in the new bucket4Merging bucket4Merging should be 0")
 	for i := len(buckets) - 1; i >= 0; i-- {
 		totNDV += buckets[i].NDV
 		res.Count += buckets[i].Count