planner: fix unique key get wrong row count when to query the NULL va…

…lue (#56117)

close #56116

pkg/planner/cardinality/row_count_index.go

@@ -251,9 +251,16 @@ func getIndexRowCountForStatsV2(sctx context.PlanContext, idx *statistics.Index,
 			if fullLen {
 				// At most 1 in this case.
 				if idx.Info.Unique {
-					totalCount++
+					if !indexRange.IsOnlyNull() {
+						totalCount++
+						if debugTrace {
+							debugTraceEndEstimateRange(sctx, 1, debugTraceUniquePoint)
+						}
+						continue
+					}
+					totalCount = float64(idx.NullCount)
 					if debugTrace {
-						debugTraceEndEstimateRange(sctx, 1, debugTraceUniquePoint)
+						debugTraceEndEstimateRange(sctx, float64(idx.NullCount), debugTraceUniquePoint)
 					}
 					continue
 				}

pkg/planner/core/casetest/index/index_test.go

@@ -220,4 +220,13 @@ func TestOrderedIndexWithIsNull(t *testing.T) {
 		"└─IndexReader_12 10.00 root  index:IndexRangeScan_11",
 		"  └─IndexRangeScan_11 10.00 cop[tikv] table:t1, index:b(b, c) range:[NULL,NULL], keep order:true, stats:pseudo",
 	))
+	// https://github.com/pingcap/tidb/issues/56116
+	tk.MustExec("create table t2(id bigint(20) DEFAULT NULL, UNIQUE KEY index_on_id (id))")
+	tk.MustExec("insert into t2 values (), (), ()")
+	tk.MustExec("analyze table t2")
+	tk.MustQuery("explain select count(*) from t2 where id is null;").Check(testkit.Rows(
+		"StreamAgg_17 1.00 root  funcs:count(Column#5)->Column#3",
+		"└─IndexReader_18 1.00 root  index:StreamAgg_9",
+		"  └─StreamAgg_9 1.00 cop[tikv]  funcs:count(1)->Column#5",
+		"    └─IndexRangeScan_16 3.00 cop[tikv] table:t2, index:index_on_id(id) range:[NULL,NULL], keep order:false"))
 }

pkg/util/ranger/types.go

@@ -142,6 +142,18 @@ func (ran *Range) isPoint(tc types.Context, regardNullAsPoint bool) bool {
 	return !ran.LowExclude && !ran.HighExclude
 }
 
+// IsOnlyNull checks if the range has [NULL, NULL] or [NULL NULL, NULL NULL] range.
+func (ran *Range) IsOnlyNull() bool {
+	for i := range ran.LowVal {
+		a := ran.LowVal[i]
+		b := ran.HighVal[i]
+		if !(a.IsNull() && b.IsNull()) {
+			return false
+		}
+	}
+	return true
+}
+
 // IsPointNonNullable returns if the range is a point without NULL.
 func (ran *Range) IsPointNonNullable(tc types.Context) bool {
 	return ran.isPoint(tc, false)