statistics: merge partition-level TopN to global-level TopN

executor/analyze.go

@@ -414,7 +414,7 @@ func (e *AnalyzeIndexExec) buildStatsFromResult(result distsql.SelectResult, nee
 				if err := cms.MergeCMSketch(cm); err != nil {
 					return nil, nil, nil, err
 				}
-				statistics.MergeTopN(topn, tmpTopN, cms, uint32(e.opts[ast.AnalyzeOptNumTopN]), false)
+				statistics.MergeTopNAndUpdateCMSketch(topn, tmpTopN, cms, uint32(e.opts[ast.AnalyzeOptNumTopN]))
 			}
 		}
 	}
@@ -1157,7 +1157,7 @@ func (e *AnalyzeFastExec) buildIndexStats(idxInfo *model.IndexInfo, collector *s
 		if err != nil {
 			return nil, nil, nil, err
 		}
-		statistics.MergeTopN(topN, curTopN, cmSketch, numTop, false)
+		statistics.MergeTopNAndUpdateCMSketch(topN, curTopN, cmSketch, numTop)
 	}
 	// Build Histogram.
 	hist, err := statistics.BuildColumnHist(e.ctx, int64(e.opts[ast.AnalyzeOptNumBuckets]), idxInfo.ID, collector, types.NewFieldType(mysql.TypeBlob), rowCount, int64(ndv), collector.NullCount*int64(scaleRatio))
@@ -1312,7 +1312,7 @@ func analyzeIndexIncremental(idxExec *analyzeIndexIncrementalExec) analyzeResult
 		cms.CalcDefaultValForAnalyze(uint64(hist.NDV))
 	}
 	if statsVer == statistics.Version2 {
-		poped := statistics.MergeTopN(topN, idxExec.oldTopN, cms, uint32(idxExec.opts[ast.AnalyzeOptNumTopN]), false)
+		poped := statistics.MergeTopNAndUpdateCMSketch(topN, idxExec.oldTopN, cms, uint32(idxExec.opts[ast.AnalyzeOptNumTopN]))
 		hist.AddIdxVals(poped)
 	}
 	result := analyzeResult{

statistics/cmsketch.go

@@ -304,43 +304,18 @@ func (c *CMSketch) queryHashValue(h1, h2 uint64) uint64 {
 	return uint64(res)
 }
 
-// MergeTopN merges the src TopN into the dst, and spilled values will be inserted into the CMSketch.
-func MergeTopN(dst, src *TopN, c *CMSketch, numTop uint32, usingMax bool) []TopNMeta {
-	if dst.TotalCount()+src.TotalCount() == 0 {
-		return nil
-	}
-	popedTopNPair := make([]TopNMeta, 0, 4)
-	counter := make(map[hack.MutableString]uint64)
-	for _, meta := range dst.TopN {
-		counter[hack.String(meta.Encoded)] += meta.Count
-	}
-	for _, meta := range src.TopN {
-		if usingMax {
-			counter[hack.String(meta.Encoded)] = mathutil.MaxUint64(counter[hack.String(meta.Encoded)], meta.Count)
-		} else {
-			counter[hack.String(meta.Encoded)] += meta.Count
-		}
-	}
-	sorted := make([]uint64, len(counter))
-	for _, cnt := range counter {
-		sorted = append(sorted, cnt)
-	}
-	sort.Slice(sorted, func(i, j int) bool {
-		return sorted[i] > sorted[j]
-	})
-	numTop = mathutil.MinUint32(uint32(len(counter)), numTop)
-	lastTopCnt := sorted[numTop-1]
-	dst.TopN = make([]TopNMeta, 0, numTop)
-	for value, cnt := range counter {
-		data := hack.Slice(string(value))
-		if cnt >= lastTopCnt {
-			dst.AppendTopN(data, cnt)
-		} else {
-			popedTopNPair = append(popedTopNPair, TopNMeta{Encoded: data, Count: cnt})
-			c.InsertBytesByCount(data, cnt)
-		}
+// MergeTopNAndUpdateCMSketch merges the src TopN into the dst, and spilled values will be inserted into the CMSketch.
+func MergeTopNAndUpdateCMSketch(dst, src *TopN, c *CMSketch, numTop uint32) []TopNMeta {
+	topNs := []*TopN{src, dst}
+	mergedTopN, popedTopNPair := MergeTopN(topNs, numTop)
+	if mergedTopN == nil {
+		// mergedTopN == nil means the total count of the input TopN are equal to zero
+		return popedTopNPair
+	}
+	dst.TopN = mergedTopN.TopN
+	for _, topNMeta := range popedTopNPair {
+		c.InsertBytesByCount(topNMeta.Encoded, topNMeta.Count)
 	}
-	dst.Sort()
 	return popedTopNPair
 }
 
@@ -652,3 +627,56 @@ func (c *TopN) RemoveVal(val []byte) {
 func NewTopN(n int) *TopN {
 	return &TopN{TopN: make([]TopNMeta, 0, n)}
 }
+
+// MergeTopN is used to merge more TopN structures to generate a new TopN struct by the given size.
+// The input parameters are multiple TopN structures to be merged and the size of the new TopN that will be generated.
+// The output parameters are the newly generated TopN structure and the remaining numbers.
+// Notice: The n can be 0. So n has no default value, we must explicitly specify this value.
+func MergeTopN(topNs []*TopN, n uint32) (*TopN, []TopNMeta) {
+	totCnt := uint64(0)
+	for _, topN := range topNs {
+		totCnt += topN.TotalCount()
+	}
+	if totCnt == 0 {
+		return nil, nil
+	}
+	// Different TopN structures may hold the same value, we have to merge them.
+	counter := make(map[hack.MutableString]uint64)
+	for _, topN := range topNs {
+		if topN.TotalCount() == 0 {
+			continue
+		}
+		for _, val := range topN.TopN {
+			counter[hack.String(val.Encoded)] += val.Count
+		}
+	}
+
+	numTop := len(counter)
+	if numTop == 0 {
+		return nil, nil
+	}
+	sorted := make([]uint64, numTop)
+	for _, cnt := range counter {
+		sorted = append(sorted, cnt)
+	}
+	sort.Slice(sorted, func(i, j int) bool {
+		return sorted[i] > sorted[j]
+	})
+	n = mathutil.MinUint32(uint32(numTop), n)
+	// lastTopCnt is the smallest value in the new TopN structure
+	lastTopCnt := sorted[numTop-1]
+
+	var finalTopN TopN
+	finalTopN.TopN = make([]TopNMeta, 0, n)
+	popedTopNPair := make([]TopNMeta, 0, uint32(numTop)-n)
+	for value, cnt := range counter {
+		data := hack.Slice(string(value))
+		if cnt >= lastTopCnt {
+			finalTopN.AppendTopN(data, cnt)
+		} else {
+			popedTopNPair = append(popedTopNPair, TopNMeta{Encoded: data, Count: cnt})
+		}
+	}
+	finalTopN.Sort()
+	return &finalTopN, popedTopNPair
+}

statistics/cmsketch_test.go

@@ -17,6 +17,7 @@ import (
 	"fmt"
 	"math"
 	"math/rand"
+	"strconv"
 	"time"
 
 	. "github.com/pingcap/check"
@@ -303,3 +304,89 @@ func (s *testStatisticsSuite) TestCMSketchCodingTopN(c *C) {
 	// do not panic
 	DecodeCMSketchAndTopN([]byte{}, rows)
 }
+
+func (s *testStatisticsSuite) TestMergeTopN(c *C) {
+	tests := []struct {
+		topnNum    int
+		n          int
+		maxTopNVal int
+		maxTopNCnt int
+	}{
+		{
+			topnNum:    10,
+			n:          5,
+			maxTopNVal: 50,
+			maxTopNCnt: 100,
+		},
+		{
+			topnNum:    1,
+			n:          5,
+			maxTopNVal: 50,
+			maxTopNCnt: 100,
+		},
+		{
+			topnNum:    5,
+			n:          5,
+			maxTopNVal: 5,
+			maxTopNCnt: 100,
+		},
+		{
+			topnNum:    5,
+			n:          5,
+			maxTopNVal: 10,
+			maxTopNCnt: 100,
+		},
+	}
+	for _, t := range tests {
+		topnNum, n := t.topnNum, t.n
+		maxTopNVal, maxTopNCnt := t.maxTopNVal, t.maxTopNCnt
+
+		// the number of maxTopNVal should be bigger than n.
+		ok := maxTopNVal >= n
+		c.Assert(ok, Equals, true)
+
+		topNs := make([]*TopN, 0, topnNum)
+		res := make(map[int]uint64)
+		rand.Seed(time.Now().Unix())
+		for i := 0; i < topnNum; i++ {
+			topN := NewTopN(n)
+			occur := make(map[int]bool)
+			for j := 0; j < n; j++ {
+				// The range of numbers in the topn structure is in [0, maxTopNVal)
+				// But there cannot be repeated occurrences of value in a topN structure.
+				randNum := rand.Intn(maxTopNVal)
+				for occur[randNum] {
+					randNum = rand.Intn(maxTopNVal)
+				}
+				occur[randNum] = true
+				tString := []byte(fmt.Sprintf("%d", randNum))
+				// The range of the number of occurrences in the topn structure is in [0, maxTopNCnt)
+				randCnt := uint64(rand.Intn(maxTopNCnt))
+				res[randNum] += randCnt
+				topNMeta := TopNMeta{tString, randCnt}
+				topN.TopN = append(topN.TopN, topNMeta)
+			}
+			topNs = append(topNs, topN)
+		}
+		topN, remainTopN := MergeTopN(topNs, uint32(n))
+		cnt := len(topN.TopN)
+		var minTopNCnt uint64
+		for _, topNMeta := range topN.TopN {
+			val, err := strconv.Atoi(string(topNMeta.Encoded))
+			c.Assert(err, IsNil)
+			c.Assert(topNMeta.Count, Equals, res[val])
+			minTopNCnt = topNMeta.Count
+		}
+		if remainTopN != nil {
+			cnt += len(remainTopN)
+			for _, remainTopNMeta := range remainTopN {
+				val, err := strconv.Atoi(string(remainTopNMeta.Encoded))
+				c.Assert(err, IsNil)
+				c.Assert(remainTopNMeta.Count, Equals, res[val])
+				ok = minTopNCnt > remainTopNMeta.Count
+				c.Assert(ok, Equals, true)
+			}
+		}
+		c.Assert(cnt, Equals, len(res))
+	}
+}

statistics/handle/handle.go

@@ -382,10 +382,15 @@ func (h *Handle) MergePartitionStats2GlobalStats(sc *stmtctx.StatementContext, i
 
 		// Merge topN. We need to merge TopN before merging the histogram.
 		// Because after merging TopN, some numbers will be left.
-		// These left numbers should be inserted into the histogram.
-		err = errors.Errorf("TODO: The merge function of the topN structure has not been implemented yet")
-		if err != nil {
-			return
+		// These remaining topN numbers will be used as a separate bucket for later histogram merging.
+		var popedTopN []statistics.TopNMeta
+		n := uint32(0)
+		for _, topN := range allTopN[i] {
+			n = mathutil.MaxUint32(n, uint32(len(topN.TopN)))
+		}
+		globalStats.TopN[i], popedTopN = statistics.MergeTopN(allTopN[i], n)
+		if len(popedTopN) != 0 {
+			// TODO: use the popedTopN as a bucket for later histogram merging.
 		}
 
 		// Merge histogram

statistics/handle/handle_test.go

@@ -706,7 +706,7 @@ func (s *testStatsSuite) TestBuildGlobalLevelStats(c *C) {
 	// Test the 'dynamic-only' mode
 	testKit.MustExec("set @@tidb_partition_prune_mode = 'dynamic-only';")
 	err := testKit.ExecToErr("analyze table t, t1;")
-	c.Assert(err.Error(), Equals, "TODO: The merge function of the topN structure has not been implemented yet")
+	c.Assert(err.Error(), Equals, "TODO: The merge function of the NDV has not been implemented yet")
 	result = testKit.MustQuery("show stats_meta where table_name = 't'").Sort()
 	c.Assert(len(result.Rows()), Equals, 3)
 	c.Assert(result.Rows()[0][5], Equals, "1")
@@ -722,7 +722,7 @@ func (s *testStatsSuite) TestBuildGlobalLevelStats(c *C) {
 	c.Assert(len(result.Rows()), Equals, 1)
 
 	err = testKit.ExecToErr("analyze table t index idx_t_ab, idx_t_b;")
-	c.Assert(err.Error(), Equals, "TODO: The merge function of the topN structure has not been implemented yet")
+	c.Assert(err.Error(), Equals, "TODO: The merge function of the NDV has not been implemented yet")
 	result = testKit.MustQuery("show stats_meta where table_name = 't'").Sort()
 	c.Assert(len(result.Rows()), Equals, 3)
 	c.Assert(result.Rows()[0][5], Equals, "1")