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https://dev.mysql.com/doc/refman/9.7/en/optimizing-innodb-storage-layout.html
Use the VARCHAR data type instead of CHAR to store variable-length strings or for columns with many NULL values. A CHAR(N) column always takes N characters to store data, even if the string is shorter or its value is NULL. Once your data reaches a ...
https://dev.mysql.com/doc/refman/9.7/en/performance-schema-accounts-table.html
This is NULL for an internal thread, or for a user session that failed to authenticate. This is NULL for an internal thread, or for a user session that failed to authenticate. The accounts table contains a row for each account that has connected to ...
https://dev.mysql.com/doc/refman/9.7/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/9.7/en/performance-schema-clone-progress-table.html
The clone_progress table shows progress information for the current or last executed cloning operation only. The stages of a cloning operation include DROP DATA, FILE COPY, PAGE_COPY, REDO_COPY, FILE_SYNC, RESTART, and RECOVERY. The table therefore ...
https://dev.mysql.com/doc/refman/9.7/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/9.7/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/9.7/en/performance-schema-obtaining-parent-events.html
FROM performance_schema.events_transactions_current AS parent INNER JOIN performance_schema.data_locks AS child WHERE parent.THREAD_ID = child.THREAD_ID AND parent.EVENT_ID < child.EVENT_ID AND ( child.EVENT_ID <= parent.END_EVENT_ID OR ...Rows of ...
https://dev.mysql.com/doc/refman/9.7/en/performance-schema-replication-applier-metrics-table.html
Until this has taken place, the value shown in this column is NULL. Until this has taken place, the value shown in this column is NULL. This table provides replication applier statistics such as wait times, wait counts, transaction counts, event ...
https://dev.mysql.com/doc/refman/9.7/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/9.7/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 ...