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MySQL 5.7 Reference Manual

25.12.15.3 Statement Summary Tables

The Performance Schema maintains tables for collecting current and recent statement events, and aggregates that information in summary tables.Section 25.12.6, “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: 370SUM_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 thesetup_instruments table:

events_statements_summary_by_account_by_event_name hasEVENT_NAME,USER, andHOST columns. Each row summarizes events for a given account (user and host combination) and event name.
events_statements_summary_by_digest hasSCHEMA_NAME andDIGEST columns. Each row summarizes events per schema and digest value. (TheDIGEST_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 theperformance_schema_digests_size system variable at server startup.
events_statements_summary_by_host_by_event_name hasEVENT_NAME andHOST columns. Each row summarizes events for a given host and event name.
events_statements_summary_by_program hasOBJECT_TYPE,OBJECT_SCHEMA, andOBJECT_NAME columns. Each row summarizes events for a given stored program (stored procedure or function, trigger, or event).
events_statements_summary_by_thread_by_event_name hasTHREAD_ID andEVENT_NAME columns. Each row summarizes events for a given thread and event name.
events_statements_summary_by_user_by_event_name hasEVENT_NAME andUSER columns. Each row summarizes events for a given user and event name.
events_statements_summary_global_by_event_name has anEVENT_NAME column. Each row summarizes events for a given event name.
prepared_statements_instances has anOBJECT_INSTANCE_BEGIN column. Each row summarizes events for a given prepared statement.

Each statement summary table has these summary columns containing aggregated values (with exceptions as noted):

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 (seeSection 25.12.15.1, “Wait Event Summary Tables”), except that the statement summary tables aggregate events fromevents_statements_current rather thanevents_waits_current.
Theprepared_statements_instances table does not have these columns.
SUM_xxx
The aggregate of the correspondingxxx column in theevents_statements_current table. For example, theSUM_LOCK_TIME andSUM_ERRORS columns in statement summary tables are the aggregates of theLOCK_TIME andERRORS columns inevents_statements_current table.

Theevents_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.

Theevents_statements_summary_by_program table has these additional summary columns:

COUNT_STATEMENTS,SUM_STATEMENTS_WAIT,MIN_STATEMENTS_WAIT,AVG_STATEMENTS_WAIT,MAX_STATEMENTS_WAIT
Statistics about nested statements invoked during stored program execution.

Theprepared_statements_instances table has these additional summary columns:

COUNT_EXECUTE,SUM_TIMER_EXECUTE,MIN_TIMER_EXECUTE,AVG_TIMER_EXECUTE,MAX_TIMER_EXECUTE
Aggregated statistics for executions of the prepared statement.

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

Forevents_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 ofevents_statements_summary_global_by_event_name. For details, seeSection 25.12.8, “Performance Schema Connection Tables”.

Statement Digest Aggregation Rules

If thestatements_digest consumer is enabled, aggregation intoevents_statements_summary_by_digest occurs as follows when a statement completes. Aggregation is based on theDIGEST value computed for the statement.

If aevents_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. TheLAST_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. TheFIRST_SEEN andLAST_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 withDIGEST =NULL, which is created if necessary. If the row is created, theFIRST_SEEN andLAST_SEEN columns are initialized with the current time. Otherwise, theLAST_SEEN column is updated with the current time.

The row withDIGEST =NULL is maintained because Performance Schema tables have a maximum size due to memory constraints. TheDIGEST =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:

ADIGEST =NULL row that has aCOUNT_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.
ADIGEST =NULL row that has aCOUNT_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 theDIGEST =NULL row would be counted using more specific rows instead. By default, the table is autosized, but if this size is too small, set theperformance_schema_digests_size system variable to a larger value at server startup.

Stored Program Instrumentation Behavior

For stored program types for which instrumentation is enabled in thesetup_objects table,events_statements_summary_by_program maintains statistics for stored programs as follows:

A row is added for an object when it is first used in the server.
The row for an object is removed when the object is dropped.
Statistics are aggregated in the row for an object as it executes.

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