Performance Schema Summary Tables When these two threads belong to the same user account, the per-account summary estimates that this account used memory in the range from 11MB to 14MB. That is, the  LOW_NUMBER_OF_BYTES_USED for the higher level aggregate is the sum of each  LOW_NUMBER_OF_BYTES_USED  (assuming the worst case). Likewise, the  HIGH_NUMBER_OF_BYTES_USED  for the higher level aggregate is the sum of each HIGH_NUMBER_OF_BYTES_USED  (assuming the worst case). 11MB is a lower estimate that can occur only if both threads hit the low usage mark at the same time. 14MB is a higher estimate that can occur only if both threads hit the high usage mark at the same time. The real memory usage for this account could have been in the range from 11.5MB to 13.5MB. For capacity planning, reporting the worst case is actually the desired behavior, as it shows what can potentially happen when sessions are uncorrelated, which is typically the case. 27.12.20.11 Error Summary Tables The Performance Schema maintains summary tables for aggregating statistical information about server errors (and warnings). For a list of server errors, see Server Error Message Reference. Collection of error information is controlled by the  error  instrument, which is enabled by default. Timing information is not collected. Each error summary table has three columns that identify the error: • ERROR_NUMBER  is the numeric error value. The value is unique. • ERROR_NAME  is the symbolic error name corresponding to the  ERROR_NUMBER  value. The value is unique. • SQLSTATE  is the SQLSTATE value corresponding to the  ERROR_NUMBER  value. The value is not necessarily unique. For example, if  ERROR_NUMBER  is 1050,  ERROR_NAME  is  ER_TABLE_EXISTS_ERROR  and  SQLSTATE  is 42S01 . Example error event summary information: mysql>  SELECT * FROM performance_schema.events_errors_summary_global_by_error WHERE SUM_ERROR_RAISED <> 0\G *************************** 1. row *************************** ERROR_NUMBER: 1064 ERROR_NAME: ER_PARSE_ERROR SQL_STATE: 42000  SUM_ERROR_RAISED: 1 SUM_ERROR_HANDLED: 0 FIRST_SEEN: 2016-06-28 07:34:02 LAST_SEEN: 2016-06-28 07:34:02 *************************** 2. row *************************** ERROR_NUMBER: 1146 ERROR_NAME: ER_NO_SUCH_TABLE SQL_STATE: 42S02  SUM_ERROR_RAISED: 2 SUM_ERROR_HANDLED: 0 FIRST_SEEN: 2016-06-28 07:34:05 LAST_SEEN: 2016-06-28 07:36:18 *************************** 3. row *************************** ERROR_NUMBER: 1317 ERROR_NAME: ER_QUERY_INTERRUPTED 5074