statistics: avoid copy twice when to decode the topn (#47563)

ref #47275
pingcap · Oct 12, 2023 · 6842efd · 6842efd
1 parent 9353c68
commit 6842efd
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Showing 2 changed files with 6 additions and 8 deletions.
diff --git a/statistics/cmsketch.go b/statistics/cmsketch.go
@@ -463,17 +463,15 @@ func DecodeCMSketchAndTopN(data []byte, topNRows []chunk.Row) (*CMSketch, *TopN,
 }
 
 // DecodeTopN decodes a TopN from the given byte slice.
-func DecodeTopN(topNRows []chunk.Row) (*TopN, error) {
-	pbTopN := make([]*tipb.CMSketchTopN, 0, len(topNRows))
+func DecodeTopN(topNRows []chunk.Row) *TopN {
+	topN := NewTopN(len(topNRows))
 	for _, row := range topNRows {
 		data := make([]byte, len(row.GetBytes(0)))
 		copy(data, row.GetBytes(0))
-		pbTopN = append(pbTopN, &tipb.CMSketchTopN{
-			Data:  data,
-			Count: row.GetUint64(1),
-		})
+		topN.AppendTopN(data, row.GetUint64(1))
 	}
-	return TopNFromProto(pbTopN), nil
+	topN.Sort()
+	return topN
 }
 
 // DecodeCMSketch encodes the given CMSketch to byte slice.

diff --git a/statistics/handle/storage/read.go b/statistics/handle/storage/read.go
@@ -134,7 +134,7 @@ func TopNFromStorage(sctx sessionctx.Context, tblID int64, isIndex int, histID i
 	if err != nil || len(rows) == 0 {
 		return nil, err
 	}
-	return statistics.DecodeTopN(rows)
+	return statistics.DecodeTopN(rows), nil
 }
 
 // FMSketchFromStorage reads FMSketch from storage