panic raised by show stats_histograms

[eurekaka]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

Add these test lines into TestShowStatsHistograms:

diff --git a/executor/show_stats_test.go b/executor/show_stats_test.go
index 05987b5..81e1771 100644
--- a/executor/show_stats_test.go
+++ b/executor/show_stats_test.go
@@ -47,6 +47,15 @@ func (s *testSuite1) TestShowStatsHistograms(c *C) {
        result = tk.MustQuery("show stats_histograms where column_name = 'a'")
        c.Assert(len(result.Rows()), Equals, 1)
        c.Assert(result.Rows()[0][3], Equals, "a")
+
+       tk.MustExec("drop table t")
+       tk.MustExec("create table t(a int, b int, c int, index idx_b(b), index idx_c_a(c, a))")
+       tk.MustExec("insert into t values(1,null,1),(2,null,2),(3,3,3),(4,null,4),(null,null,null)")
+       res := tk.MustQuery("show stats_histograms where table_name = 't'")
+       c.Assert(len(res.Rows()), Equals, 0)
+       tk.MustExec("analyze table t index idx_b")
+       res = tk.MustQuery("show stats_histograms where table_name = 't' and column_name = 'idx_b'")
+       c.Assert(len(res.Rows()), Equals, 1)
 }

then run go test under executor package.

2. What did you expect to see?

Test result should be either OK or FAIL, not PANIC.

3. What did you see instead?

----------------------------------------------------------------------
PANIC: show_stats_test.go:37: testSuite1.TestShowStatsHistograms

... Panic: runtime error: invalid memory address or nil pointer dereference (PC=0xB8581A)

/usr/local/go/src/runtime/panic.go:513
  in gopanic
/usr/local/go/src/runtime/panic.go:82
  in panicmem
/usr/local/go/src/runtime/signal_unix.go:390
  in sigpanic
/root/go/src/github.com/pingcap/tidb/statistics/histogram.go:112
  in Column.AvgColSize
show_stats.go:85
  in ShowExec.appendTableForStatsHistograms
show_stats.go:69
  in ShowExec.fetchShowStatsHistogram
show.go:147
  in ShowExec.fetchAll
show.go:74
  in ShowExec.Next
executor.go:928
  in SelectionExec.Next
adapter.go:104
  in recordSet.Next
/root/go/src/github.com/pingcap/tidb/session/tidb.go:264
  in GetRows4Test
/root/go/src/github.com/pingcap/tidb/util/testkit/testkit.go:211
  in TestKit.ResultSetToResult
/root/go/src/github.com/pingcap/tidb/util/testkit/testkit.go:185
  in TestKit.MustQuery
show_stats_test.go:57
  in testSuite1.TestShowStatsHistograms
/usr/local/go/src/reflect/value.go:308
  in Value.Call
/root/go/pkg/mod/github.com/pingcap/[email protected]/check.go:836
  in suiteRunner.forkTest.func1
/root/go/pkg/mod/github.com/pingcap/[email protected]/check.go:730
  in suiteRunner.forkCall.func1
/usr/local/go/src/runtime/asm_amd64.s:1333
  in goexit

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

Release Version: v3.0.0-beta-124-g511a80b-dirty
Git Commit Hash: 511a80bae4d37b65b7f9e41e23f700c9b4c79584
Git Branch: master
UTC Build Time: 2019-02-23 11:33:57
GoVersion: go version go1.11.2 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false