code: func upgradeToVer25(s Session) {
  	sql := fmt.Sprintf("UPDATE HIGH_PRIORITY %[1]s.%[2]s SET VARIABLE_VALUE = '%[4]d' WHERE VARIABLE_NAME = '%[3]s' AND VARIABLE_VALUE < %[4]d",
  		mysql.SystemDB, mysql.GlobalVariablesTable, variable.TiDBMaxChunkSize, variable.DefInitChunkSize)
  	mustExecute(s, sql)
  }
docs: // upgradeToVer25 updates tidb_max_chunk_size to new low bound value 32 if previous value is small than 32.