From bebd5c59e4c8eb0e4e626ec67914766e270608db Mon Sep 17 00:00:00 2001
From: Haibin Xie <lambdax.tyler@gmail.com>
Date: Mon, 10 Jul 2017 15:40:46 +0800
Subject: [PATCH] statistics: fix bug when calculate lowerbound (#3677)

---
 statistics/builder.go         |  7 ++++---
 statistics/histogram.go       |  2 +-
 statistics/statistics_test.go | 18 ++++++++++++++++--
 3 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/statistics/builder.go b/statistics/builder.go
index 7fac5b5ebe..8ad8c39a67 100644
--- a/statistics/builder.go
+++ b/statistics/builder.go
@@ -74,6 +74,9 @@ func build4SortedColumn(ctx context.Context, numBuckets, id int64, records ast.R
 			hg.Buckets[bucketIdx].Count++
 			hg.Buckets[bucketIdx].UpperBound = data
 			hg.Buckets[bucketIdx].Repeats = 1
+			if bucketIdx == 0 && hg.Buckets[0].Count == 1 {
+				hg.Buckets[0].LowerBound = data
+			}
 			hg.NDV++
 		} else {
 			// All buckets are full, we should merge buckets.
@@ -137,6 +140,7 @@ func BuildColumn(ctx context.Context, numBuckets, id int64, ndv int64, count int
 	}
 	bucketIdx := 0
 	var lastCount int64
+	hg.Buckets[0].LowerBound = samples[0]
 	for i := int64(0); i < int64(len(samples)); i++ {
 		cmp, err := hg.Buckets[bucketIdx].UpperBound.CompareDatum(sc, samples[i])
 		if err != nil {
@@ -158,9 +162,6 @@ func BuildColumn(ctx context.Context, numBuckets, id int64, ndv int64, count int
 			hg.Buckets[bucketIdx].Count = int64(totalCount)
 			hg.Buckets[bucketIdx].UpperBound = samples[i]
 			hg.Buckets[bucketIdx].Repeats = int64(ndvFactor)
-			if bucketIdx == 0 {
-				hg.Buckets[bucketIdx].LowerBound = samples[i]
-			}
 		} else {
 			lastCount = hg.Buckets[bucketIdx].Count
 			// The bucket is full, store the item in the next bucket.
diff --git a/statistics/histogram.go b/statistics/histogram.go
index 10e99dd94d..8c8b12d0c0 100644
--- a/statistics/histogram.go
+++ b/statistics/histogram.go
@@ -238,7 +238,7 @@ func (hg *Histogram) lessRowCount(sc *variable.StatementContext, value types.Dat
 	if err != nil {
 		return 0, errors.Trace(err)
 	}
-	if c < 0 {
+	if c <= 0 {
 		return prevCount, nil
 	}
 	return (prevCount + lessThanBucketValueCount) / 2, nil
diff --git a/statistics/statistics_test.go b/statistics/statistics_test.go
index db7aa38512..b8940bfdd3 100644
--- a/statistics/statistics_test.go
+++ b/statistics/statistics_test.go
@@ -70,7 +70,11 @@ func (r *recordSet) Close() error {
 func (s *testStatisticsSuite) SetUpSuite(c *C) {
 	s.count = 100000
 	samples := make([]types.Datum, 10000)
-	start := 1000 // 1000 values is null
+	start := 1000
+	samples[0].SetInt64(0)
+	for i := 1; i < start; i++ {
+		samples[i].SetInt64(2)
+	}
 	for i := start; i < len(samples); i++ {
 		samples[i].SetInt64(int64(i))
 	}
@@ -90,6 +94,10 @@ func (s *testStatisticsSuite) SetUpSuite(c *C) {
 		count:  s.count,
 		cursor: 0,
 	}
+	rc.data[0].SetInt64(0)
+	for i := 1; i < start; i++ {
+		rc.data[i].SetInt64(2)
+	}
 	for i := int64(start); i < rc.count; i++ {
 		rc.data[i].SetInt64(int64(i))
 	}
@@ -152,6 +160,9 @@ func (s *testStatisticsSuite) TestBuild(c *C) {
 	count, err = col.betweenRowCount(sc, types.NewIntDatum(3000), types.NewIntDatum(3500))
 	c.Check(err, IsNil)
 	c.Check(int(count), Equals, 5075)
+	count, err = col.lessRowCount(sc, types.NewIntDatum(1))
+	c.Check(err, IsNil)
+	c.Check(int(count), Equals, 9)
 
 	tblCount, col, err := BuildIndex(ctx, bucketCount, 1, ast.RecordSet(s.rc))
 	c.Check(err, IsNil)
@@ -165,6 +176,9 @@ func (s *testStatisticsSuite) TestBuild(c *C) {
 	count, err = col.betweenRowCount(sc, encodeKey(types.NewIntDatum(30000)), encodeKey(types.NewIntDatum(35000)))
 	c.Check(err, IsNil)
 	c.Check(int(count), Equals, 4618)
+	count, err = col.lessRowCount(sc, encodeKey(types.NewIntDatum(0)))
+	c.Check(err, IsNil)
+	c.Check(int(count), Equals, 0)
 
 	tblCount, col, err = BuildPK(ctx, bucketCount, 4, ast.RecordSet(s.pk))
 	c.Check(err, IsNil)
@@ -186,7 +200,7 @@ func (s *testStatisticsSuite) TestBuild(c *C) {
 	c.Check(int(count), Equals, 100000)
 	count, err = col.lessAndEqRowCount(sc, types.Datum{})
 	c.Check(err, IsNil)
-	c.Check(int(count), Equals, 256)
+	c.Check(int(count), Equals, 0)
 	count, err = col.greaterRowCount(sc, types.NewIntDatum(1001))
 	c.Check(err, IsNil)
 	c.Check(int(count), Equals, 99231)