From a604245298964a1139d1ddec5d8657eeee26ab80 Mon Sep 17 00:00:00 2001
From: Kenan Yao <cauchy1992@gmail.com>
Date: Mon, 28 Jun 2021 11:44:29 +0800
Subject: [PATCH] statistics: skip dumping nil histograms for virtual columns
 in analyze

---
 executor/analyze.go              |  5 ++++-
 statistics/handle/handle.go      |  4 ++++
 statistics/handle/handle_test.go | 12 ++++++++++++
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/executor/analyze.go b/executor/analyze.go
index 889c00297b4b4..c62404331c22c 100644
--- a/executor/analyze.go
+++ b/executor/analyze.go
@@ -596,6 +596,10 @@ func analyzeColumnsPushdown(colExec *AnalyzeColumnsExec) *statistics.AnalyzeResu
 			}
 		}
 		idxNDVPushDownCh := make(chan analyzeIndexNDVTotalResult, 1)
+		// subIndexWorkerWg is better to be initialized in handleNDVForSpecialIndexes, however if we do so, golang would
+		// report unexpected/unreasonable data race error on subIndexWorkerWg when running TestAnalyzeVirtualCol test
+		// case with `-race` flag now.
+		colExec.subIndexWorkerWg = &sync.WaitGroup{}
 		go colExec.handleNDVForSpecialIndexes(specialIndexes, idxNDVPushDownCh)
 		count, hists, topns, fmSketches, extStats, err := colExec.buildSamplingStats(ranges, collExtStats, specialIndexesOffsets, idxNDVPushDownCh)
 		if err != nil {
@@ -1008,7 +1012,6 @@ func (e *AnalyzeColumnsExec) handleNDVForSpecialIndexes(indexInfos []*model.Inde
 		statistics.AddNewAnalyzeJob(task.job)
 	}
 	resultsCh := make(chan *statistics.AnalyzeResults, len(tasks))
-	e.subIndexWorkerWg = &sync.WaitGroup{}
 	e.subIndexWorkerWg.Add(statsConcurrncy)
 	for i := 0; i < statsConcurrncy; i++ {
 		go e.subIndexWorkerForNDV(taskCh, resultsCh, i == 0)
diff --git a/statistics/handle/handle.go b/statistics/handle/handle.go
index 9bd279d9539f5..bbd500e8e7c38 100644
--- a/statistics/handle/handle.go
+++ b/statistics/handle/handle.go
@@ -1001,6 +1001,10 @@ func (h *Handle) SaveTableStatsToStorage(results *statistics.AnalyzeResults, nee
 	// 2. Save histograms.
 	for _, result := range results.Ars {
 		for i, hg := range result.Hist {
+			// It's normal virtual column, skip it.
+			if hg == nil {
+				continue
+			}
 			var cms *statistics.CMSketch
 			if results.StatsVer != statistics.Version2 {
 				cms = result.Cms[i]
diff --git a/statistics/handle/handle_test.go b/statistics/handle/handle_test.go
index ab00916ad1b11..001bacac3e4a5 100644
--- a/statistics/handle/handle_test.go
+++ b/statistics/handle/handle_test.go
@@ -759,6 +759,18 @@ func (s *testStatsSuite) TestCorrelation(c *C) {
 	c.Assert(result.Rows()[0][9], Equals, "0")
 }
 
+func (s *testStatsSuite) TestAnalyzeVirtualCol(c *C) {
+	defer cleanEnv(c, s.store, s.do)
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(a int, b int generated always as (-a) virtual, c int generated always as (-a) stored, index (c))")
+	tk.MustExec("insert into t(a) values(2),(1),(1),(3),(NULL)")
+	tk.MustExec("set @@tidb_analyze_version = 2")
+	tk.MustExec("analyze table t")
+	c.Assert(len(tk.MustQuery("show stats_histograms where table_name ='t'").Rows()), Equals, 3)
+}
+
 func (s *testStatsSuite) TestShowGlobalStats(c *C) {
 	defer cleanEnv(c, s.store, s.do)
 	tk := testkit.NewTestKit(c, s.store)