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https://dev.mysql.com/doc/refman/5.7/en/performance-schema-atom-molecule-events.html
For example, a row fetch might result in rows like this: Row# EVENT_NAME TIMER_START TIMER_END ---- ---------- ----------- --------- 1 wait/io/file/myisam/dfile 10001 10002 2 wait/io/table/sql/handler 10000 NULL The row fetch causes a file read. In ... For a table I/O event, there are usually two rows in events_waits_current, not ...
https://dev.mysql.com/doc/refman/5.7/en/performance-schema-events-stages-current-table.html
END_EVENT_ID This column is set to NULL when the event starts and updated to the thread current event number when the event ends. If an event is produced from an instrument that has TIMED = NO, timing information is not collected, and TIMER_START, ...The table stores one row per thread showing the current status of the thread's most recent monitored stage event, so there is no system variable for configuring the table ...
https://dev.mysql.com/doc/refman/5.7/en/performance-schema-mutex-instances-table.html
LOCKED_BY_THREAD_ID When a thread currently has a mutex locked, LOCKED_BY_THREAD_ID is the THREAD_ID of the locking thread, otherwise it is NULL. When a thread unlocks a mutex, mutex_instances shows that the mutex now has no owner (the THREAD_ID ...
https://dev.mysql.com/doc/refman/5.7/en/performance-schema-performance-timers-table.html
The rows that do not contain NULL indicate which timers you can use in setup_timers. For an explanation of how event timing occurs, see Section 25.4.1, “Performance Schema Event Timing”. Note As of MySQL 5.7.21, the Performance Schema ...
https://dev.mysql.com/doc/refman/5.7/en/performance-schema-stage-tables.html
The Performance Schema instruments stages, which are steps during the statement-execution process, such as parsing a statement, opening a table, or performing a filesort operation. Stages correspond to the thread states displayed by SHOW ...
https://dev.mysql.com/doc/refman/5.7/en/performance-schema-table-wait-summary-tables.html
A value of NULL means that table I/O used no index. The following sections describe the table I/O and lock wait summary tables: table_io_waits_summary_by_index_usage: Table I/O waits per index table_io_waits_summary_by_table: Table I/O waits per ...
https://dev.mysql.com/doc/refman/5.7/en/primary-key-optimization.html
Query performance benefits from the NOT NULL optimization, because it cannot include any NULL values. The primary key for a table represents the column or set of columns that you use in your most vital queries. With the InnoDB storage engine, the ...
https://dev.mysql.com/doc/refman/5.7/en/processlist-access.html
db is the default database for the thread, or NULL if none has been selected. Info indicates the statement the thread is executing, or NULL if it is executing no statement. The following discussion enumerates the sources of process information, the ...
https://dev.mysql.com/doc/refman/5.7/en/replication-administration-status.html
row *************************** Id: 10 User: root Host: replica1:58371 db: NULL Command: Binlog Dump Time: 777 State: Has sent all binlog to slave; waiting for binlog to be updated Info: NULL Because it is the replica that drives the replication ...
https://dev.mysql.com/doc/refman/5.7/en/rewriter-query-rewrite-plugin-reference.html
If no error occurs when the rules table is loaded into memory, the plugin sets the message column to NULL. A non-NULL value indicates an error and the column contents are the error message. Users communicate with the plugin by modifying the set of ...