update

Signed-off-by: Weizhen Wang <[email protected]>

Author: hawkingrei

pkg/expression/grouping_sets.go

@@ -242,8 +242,8 @@ func (gs GroupingSet) AllColIDs() *intset.FastIntSet {
 }
 
 // ExtractCols is used to extract basic columns from one grouping set.
-func (gs GroupingSet) ExtractCols() []*Column {
-	cols := make([]*Column, 0, len(gs))
+// the param cols is used for reuse the slice.
+func (gs GroupingSet) ExtractCols(cols []*Column) []*Column {
 	for _, groupingExprs := range gs {
 		for _, one := range groupingExprs {
 			cols = append(cols, one.(*Column))

pkg/planner/core/task.go

@@ -1582,11 +1582,13 @@ func scaleStats4GroupingSets(p *physicalop.PhysicalHashAgg, groupingSets express
 		}
 	}
 	sumNDV := float64(0)
+	groupingSetCols := make([]*expression.Column, 0, 4)
 	for _, groupingSet := range groupingSets {
 		// for every grouping set, pick its cols out, and combine with normal group cols to get the ndv.
-		groupingSetCols := groupingSet.ExtractCols()
+		groupingSetCols = groupingSet.ExtractCols(groupingSetCols)
 		groupingSetCols = append(groupingSetCols, normalGbyCols...)
 		ndv, _ := cardinality.EstimateColsNDVWithMatchedLen(p.SCtx(), groupingSetCols, childSchema, childStats)
+		groupingSetCols = groupingSetCols[:0]
 		sumNDV += ndv
 	}
 	// After group operator, all same rows are grouped into one row, that means all