22.12.9.3 Statement Summary Tables

The Performance Schema maintains tables for collecting current and recent statement events, and aggregates that information in summary tables. Section 22.12.6, “Performance Schema Statement Event Tables” describes the events on which statement summaries are based. See that discussion for information about the content of statement events, the current and historical statement event tables, and how to control statement event collection, which is partially disabled by default.

Example statement event summary information:

mysql> SELECT *
       FROM performance_schema.events_statements_summary_global_by_event_name\G
*************************** 1. row ***************************
                 EVENT_NAME: statement/sql/select
                 COUNT_STAR: 25
             SUM_TIMER_WAIT: 1535983999000
             MIN_TIMER_WAIT: 209823000
             AVG_TIMER_WAIT: 61439359000
             MAX_TIMER_WAIT: 1363397650000
              SUM_LOCK_TIME: 20186000000
                 SUM_ERRORS: 0
               SUM_WARNINGS: 0
          SUM_ROWS_AFFECTED: 0
              SUM_ROWS_SENT: 388
          SUM_ROWS_EXAMINED: 370
SUM_CREATED_TMP_DISK_TABLES: 0
     SUM_CREATED_TMP_TABLES: 0
       SUM_SELECT_FULL_JOIN: 0
 SUM_SELECT_FULL_RANGE_JOIN: 0
           SUM_SELECT_RANGE: 0
     SUM_SELECT_RANGE_CHECK: 0
            SUM_SELECT_SCAN: 6
      SUM_SORT_MERGE_PASSES: 0
             SUM_SORT_RANGE: 0
              SUM_SORT_ROWS: 0
              SUM_SORT_SCAN: 0
          SUM_NO_INDEX_USED: 6
     SUM_NO_GOOD_INDEX_USED: 0
...

Each statement summary table has one or more grouping columns to indicate how the table aggregates events. Event names refer to names of event instruments in the setup_instruments table:

events_statements_summary_by_account_by_event_name has EVENT_NAME, USER, and HOST columns. Each row summarizes events for a given account (user and host combination) and event name.
events_statements_summary_by_digest has SCHEMA_NAME and DIGEST columns. Each row summarizes events per schema and digest value. (The DIGEST_TEXT column contains the corresponding normalized statement digest text, but is neither a grouping nor a summary column.)
The maximum number of rows in the table is autosized at server startup. To set this maximum explicitly, set the performance_schema_digests_size system variable at server startup.
events_statements_summary_by_host_by_event_name has EVENT_NAME and HOST columns. Each row summarizes events for a given host and event name.
events_statements_summary_by_thread_by_event_name has THREAD_ID and EVENT_NAME columns. Each row summarizes events for a given thread and event name.
events_statements_summary_by_user_by_event_name has EVENT_NAME and USER columns. Each row summarizes events for a given user and event name.
events_statements_summary_global_by_event_name has an EVENT_NAME column. Each row summarizes events for a given event name.

Each statement summary table has these summary columns containing aggregated values:

COUNT_STAR, SUM_TIMER_WAIT, MIN_TIMER_WAIT, AVG_TIMER_WAIT, MAX_TIMER_WAIT
These columns are analogous to the columns of the same names in the wait event summary tables (see Section 22.12.9.1, “Wait Event Summary Tables”), except that the statement summary tables aggregate events from events_statements_current rather than events_waits_current.
SUM_xxx
The aggregate of the corresponding xxx column in the events_statements_current table. For example, the SUM_LOCK_TIME and SUM_ERRORS columns in statement summary tables are the aggregates of the LOCK_TIME and ERRORS columns in events_statements_current table.

The events_statements_summary_by_digest table has these additional summary columns:

FIRST_SEEN, LAST_SEEN
Timestamps indicating when statements with the given digest value were first seen and most recently seen.

TRUNCATE TABLE is permitted for statement summary tables. It has these effects:

For events_statements_summary_by_digest, it removes the rows.
For other summary tables not aggregated by account, host, or user, truncation resets the summary columns to zero rather than removing rows.
For other summary tables aggregated by account, host, or user, truncation removes rows for accounts, hosts, or users with no connections, and resets the summary columns to zero for the remaining rows.

In addition, each statement summary table that is aggregated by account, host, user, or thread is implicitly truncated by truncation of the connection table on which it depends, or truncation of events_statements_summary_global_by_event_name. For details, see Section 22.12.7, “Performance Schema Connection Tables”.

Statement Digest Aggregation Rules

If the statements_digest consumer is enabled, aggregation into events_statements_summary_by_digest occurs as follows when a statement completes. Aggregation is based on the DIGEST value computed for the statement.

If a events_statements_summary_by_digest row already exists with the digest value for the statement that just completed, statistics for the statement are aggregated to that row. The LAST_SEEN column is updated to the current time.
If no row has the digest value for the statement that just completed, and the table is not full, a new row is created for the statement. The FIRST_SEEN and LAST_SEEN columns are initialized with the current time.
If no row has the statement digest value for the statement that just completed, and the table is full, the statistics for the statement that just completed are added to a special “catch-all” row with DIGEST = NULL, which is created if necessary. If the row is created, the FIRST_SEEN and LAST_SEEN columns are initialized with the current time. Otherwise, the LAST_SEEN column is updated with the current time.

The row with DIGEST = NULL is maintained because Performance Schema tables have a maximum size due to memory constraints. The DIGEST = NULL row permits digests that do not match other rows to be counted even if the summary table is full, using a common “other” bucket. This row helps you estimate whether the digest summary is representative:

A DIGEST = NULL row that has a COUNT_STAR value that represents 5% of all digests shows that the digest summary table is very representative; the other rows cover 95% of the statements seen.
A DIGEST = NULL row that has a COUNT_STAR value that represents 50% of all digests shows that the digest summary table is not very representative; the other rows cover only half the statements seen. Most likely the DBA should increase the maximum table size so that more of the rows counted in the DIGEST = NULL row would be counted using more specific rows instead. To do this, set the performance_schema_digests_size system variable to a larger value at server startup. The default size is 200.