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statistic: fix panic when building topN (#47928) #47990

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merged 2 commits into from
Oct 31, 2023
Merged

statistic: fix panic when building topN (#47928) #47990

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6199ae8
This is an automated cherry-pick of #47928
Rustin170506 Oct 25, 2023
9fe9fe0
Fix
Rustin170506 Oct 31, 2023
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38 changes: 37 additions & 1 deletion statistics/builder.go
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Original file line number Diff line number Diff line change
Expand Up @@ -24,7 +24,9 @@ import (
"github.com/pingcap/tidb/types"
"github.com/pingcap/tidb/util/codec"
"github.com/pingcap/tidb/util/collate"
"github.com/pingcap/tidb/util/logutil"
"github.com/pingcap/tidb/util/memory"
"go.uber.org/zap"
)

// SortedBuilder is used to build histograms for PK and index.
Expand Down Expand Up @@ -372,12 +374,46 @@ func BuildHistAndTopN(
if err != nil {
return nil, nil, errors.Trace(err)
}
// For debugging invalid sample data.
var (
foundTwice bool
firstTimeSample types.Datum
)
for j := 0; j < len(topNList); j++ {
if bytes.Equal(sampleBytes, topNList[j].Encoded) {
// find the same value in topn: need to skip over this value in samples
// This should never happen, but we met this panic before, so we add this check here.
// See: https://github.com/pingcap/tidb/issues/35948
if foundTwice {
datumString, err := firstTimeSample.ToString()
if err != nil {
logutil.BgLogger().With(
zap.String("category", "stats"),
).Error("try to convert datum to string failed", zap.Error(err))
}

logutil.BgLogger().With(
zap.String("category", "stats"),
).Warn(
"invalid sample data",
zap.Bool("isColumn", isColumn),
zap.Int64("columnID", id),
zap.String("datum", datumString),
zap.Binary("sampleBytes", sampleBytes),
zap.Binary("topNBytes", topNList[j].Encoded),
)
// NOTE: if we don't return here, we may meet panic in the following code.
// The i may decrease to a negative value.
// We haven't fix the issue here, because we don't know how to
// remove the invalid sample data from the samples.
break
}
// First time to find the same value in topN: need to record the sample data for debugging.
firstTimeSample = samples[i].Value
// Found the same value in topn: need to skip over this value in samples.
copy(samples[i:], samples[uint64(i)+topNList[j].Count:])
samples = samples[:uint64(len(samples))-topNList[j].Count]
i--
foundTwice = true
continue
}
}
Expand Down