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https://dev.mysql.com/doc/refman/5.7/en/performance-schema-timing.html
The rows that do not contain NULL indicate which timers you can use in setup_timers. The timers in setup_timers that you can use are those that do not have NULL in the other columns. If the values associated with a given timer name are NULL, that ...
https://dev.mysql.com/doc/refman/5.7/en/replication-options-replica.html
This section explains the server options and system variables that apply to replicas and contains the following: Startup Options for Replicas Options for Logging Replica Status to Tables System Variables Used on Replicas Specify the options either ...
https://dev.mysql.com/doc/refman/5.7/en/json-search-functions.html
Returns NULL if any argument is NULL, or if the path argument does not identify a section of the target document. Returns NULL if any argument is NULL or no paths locate a value in the document. row *************************** id: 1 select_type: ...
https://dev.mysql.com/doc/refman/5.7/en/date-and-time-type-syntax.html
Also, any TIMESTAMP not explicitly declared as NOT NULL permits NULL values. You can also set any TIMESTAMP column to the current date and time by assigning it a NULL value, unless it has been defined with the NULL attribute to permit NULL values.
https://dev.mysql.com/doc/refman/5.7/en/group-by-handling.html
The query is invalid if name is not a primary key of t or a unique NOT NULL column. SQL-92 and earlier does not permit queries for which the select list, HAVING condition, or ORDER BY list refer to nonaggregated columns that are not named in the ...
https://dev.mysql.com/doc/refman/5.7/en/create-index.html
A UNIQUE index permits multiple NULL values for columns that can contain NULL. Otherwise, _rowid refers to the column in the first UNIQUE NOT NULL index if that index consists of a single integer column. If the first UNIQUE NOT NULL index does not ... CREATE [UNIQUE | FULLTEXT | SPATIAL] INDEX index_name [index_type] ON tbl_name (key_part,...) [index_option] [algorithm_option | lock_option] ...
https://dev.mysql.com/doc/refman/5.7/en/optimizing-myisam-bulk-data-loading.html
These performance tips supplement the general guidelines for fast inserts in Section 8.2.4.1, “Optimizing INSERT Statements”. For a MyISAM table, you can use concurrent inserts to add rows at the same time that SELECT statements are running, if ...
https://dev.mysql.com/doc/refman/5.7/en/information-schema-files-table.html
The value is NULL for all data files except for predefined system tablespace data files. A NULL value for a predefined system tablespace data file indicates that a file size limit was not defined explicitly. Where no value is provided by the ... The ...
https://dev.mysql.com/doc/refman/5.7/en/glossary.html
It provides useful information for the query optimizer, because the column is known to be not null and with unique values. A column can be subject to a unique constraint, a NOT NULL constraint, or both. See Also cardinality, foreign key, index, NOT ... These terms are commonly used in information about the MySQL database ...
https://dev.mysql.com/doc/refman/5.7/en/performance-schema-statement-digests.html
If the table becomes full, the Performance Schema groups statements that have SCHEMA_NAME and DIGEST values not matching existing values in the table in a special row with SCHEMA_NAME and DIGEST set to NULL. The MySQL server is capable of ...