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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 ...
https://dev.mysql.com/doc/refman/5.7/en/sys-diagnostics.html
Use NULL to collect data for the default of 60 seconds. Creates a report of the current server status for diagnostic purposes. This procedure disables binary logging during its execution by manipulating the session value of the sql_log_bin system ...
https://dev.mysql.com/doc/refman/5.7/en/sys-processlist.html
db The default database for the thread, or NULL if there is none. current_statement The statement the thread is executing, or NULL if it is not executing any statement. The MySQL process list indicates the operations currently being performed by ...
https://dev.mysql.com/doc/refman/5.7/en/sys-ps-is-thread-instrumented.html
Returns YES or NO to indicate whether Performance Schema instrumentation for a given connection ID is enabled, UNKNOWN if the ID is unknown, or NULL if the ID is NULL. This is a value of the type given in the PROCESSLIST_ID column of the ...
https://dev.mysql.com/doc/refman/5.7/en/charset-metadata.html
Thus column names, database names, user names, version names, and most of the string results from SHOW are metadata. If character_set_results is set to NULL, no conversion is performed and the server returns metadata using its original character set ... Metadata is “the data about the data.” Anything that describes the database—as opposed to being the contents of the database—is ...
https://dev.mysql.com/doc/refman/5.7/en/myisamchk-general-options.html
--debug=debug_options, -# debug_options Command-Line Format --debug[=debug_options] Type String Default Value d:t:o,/tmp/myisamchk.trace Write a debugging log. --defaults-group-suffix=str Command-Line Format --defaults-group-suffix=str Type String ... The options described in this section can be used for any type of table maintenance operation performed by ...