stats: fix panic when log detailed stats info

[alivxxx]

What problem does this PR solve?

panic: runtime error: index out of range                                 
                                                                         
goroutine 14313 [running]:                                               
github.com/pingcap/tidb/util/chunk.(*column).isNull(...)                 
        /home/jenkins/workspace/build_tidb_master/go/src/github.com/pingcap/tidb/util/chunk/column.go:71
github.com/pingcap/tidb/util/chunk.Row.IsNull(...)                       
        /home/jenkins/workspace/build_tidb_master/go/src/github.com/pingcap/tidb/util/chunk/row.go:222
github.com/pingcap/tidb/util/chunk.Row.GetDatum(0xc000021f50, 0xffffffffffffffff, 0x0, 0xc0004a5810, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
        /home/jenkins/workspace/build_tidb_master/go/src/github.com/pingcap/tidb/util/chunk/row.go:174 +0xbd0
github.com/pingcap/tidb/statistics.(*Histogram).GetUpper(0xc00058cd80, 0xffffffffffffffff, 0x13503b6)
        /home/jenkins/workspace/build_tidb_master/go/src/github.com/pingcap/tidb/statistics/histogram.go:102 +0x82
github.com/pingcap/tidb/statistics.(*Histogram).bucketToString(0xc00058cd80, 0xffffffffffffffff, 0x0, 0x0, 0xc005c97780)
        /home/jenkins/workspace/build_tidb_master/go/src/github.com/pingcap/tidb/statistics/histogram.go:356 +0x4d
github.com/pingcap/tidb/statistics.formatBuckets(0xc00058cd80, 0xffffffffffffffff, 0xffffffffffffffff, 0x0, 0xb, 0x12)
        /home/jenkins/workspace/build_tidb_master/go/src/github.com/pingcap/tidb/statistics/feedback.go:767 +0x3e5
github.com/pingcap/tidb/statistics.colRangeToStr(0xc00058cd80, 0xc002461c20, 0xffffffffffffffff, 0x3ff448be405987b2, 0xc000345498, 0x0)
        /home/jenkins/workspace/build_tidb_master/go/src/github.com/pingcap/tidb/statistics/feedback.go:781 +0x14b
github.com/pingcap/tidb/statistics.logForIndex(0xc0064a1890, 0x25, 0xc0057c0060, 0xc00058db00, 0xc000734f08, 0x1, 0x1, 0xc00162ffa8, 0x1, 0x1, ...)
        /home/jenkins/workspace/build_tidb_master/go/src/github.com/pingcap/tidb/statistics/feedback.go:858 +0xb72

What is changed and how it works?

Only format buckets when there are buckes in the histogram, or it will panic when get the bucket boundaries. If there is no buckets, we only log the expected count.

Check List

Tests

• Unit test

Code changes

• Has unexported function/method change

Side effects

• None

Related changes

• None

PTAL @jackysp @coocood @zz-jason @winoros

[jackysp]

LGTM

[winoros]

lgtm

[alivxxx]

/run-all-tests

[shenli]

Do we need to cherry-pick it to the release-2.0?

[alivxxx]

@shenli No, because release 2.0 does not log the stats info for feedback.