Slow log query's parse concurrency is not stable

[yibin87]

Enhancement

There are two types of concurency for parsing slow query log, decided by whether Desc attribute is true:

tidb/pkg/executor/slow_query.go

Lines 451 to 455 in d8098b9

	if !e.extractor.Desc {
	logs, err = e.getBatchLog(ctx, reader, &offset, logNum)
	} else {
	logs, err = e.getBatchLogForReversedScan(ctx, reader, &offset, logNum)
	}

• When Desc is true, the concurrency will be DistSQLScanConcurrency
• When false, the concurrency will be 1, due to the parsed log's first dimension length is always <= 1:
  

  tidb/pkg/executor/slow_query.go

  Line 316 in d8098b9

  log := make([]string, 0, num)

  
  

  tidb/pkg/executor/slow_query.go

  Line 348 in d8098b9

  return [][]string{log}, err

[yibin87]

/assign @yibin87