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https://dev.mysql.com/doc/refman/8.0/en/subquery-optimization-with-exists.html
WHERE subquery_where AND (outer_expr=inner_expr OR inner_expr IS NULL)) The need to evaluate the extra IS NULL condition is why MySQL has the ref_or_null access method: mysql> EXPLAIN SELECT outer_expr IN (SELECT t2.maybe_null_key FROM t2, t3 WHERE ... Certain optimizations are applicable to comparisons that use the IN (or =ANY) operator to test subquery ...
https://dev.mysql.com/doc/refman/8.0/en/with.html
For recursive CTEs, EXPLAIN output rows for recursive SELECT parts display Recursive in the Extra column. Cost estimates displayed by EXPLAIN represent cost per iteration, which might differ considerably from total cost. A common table expression ...
https://dev.mysql.com/doc/refman/8.0/en/derived-condition-pushdown-optimization.html
MySQL 8.0.22 and later supports derived condition pushdown for eligible subqueries. When a derived table cannot be merged into the outer query (for example, if the derived table uses aggregation), pushing the outer WHERE condition down to the ...
https://dev.mysql.com/doc/refman/8.0/en/generated-column-index-optimizations.html
You can see this using EXPLAIN: mysql> EXPLAIN SELECT * FROM t1 WHERE f1 + 1 > 9\G *************************** 1. That is also apparent in the rewritten query available in the extended EXPLAIN information displayed by SHOW WARNINGS: mysql> SHOW ...
https://dev.mysql.com/doc/refman/8.0/en/index-hints.html
These hints are useful if EXPLAIN shows that MySQL is using the wrong index from the list of possible indexes. Index hints give the optimizer information about how to choose indexes during query processing. Index hints, described here, differ from ...
https://dev.mysql.com/doc/refman/8.0/en/index-merge-optimization.html
In EXPLAIN output, the Index Merge method appears as index_merge in the type column. The Index Merge access method has several algorithms, which are displayed in the Extra field of EXPLAIN output: Using intersect(...) Using union(...) Using ... The ...
https://dev.mysql.com/doc/refman/8.0/en/invisible-indexes.html
Using the SET_VAR optimizer hint to update the value of optimizer_switch temporarily, you can enable invisible indexes for the duration of a single query only, like this: mysql> EXPLAIN SELECT /*+ SET_VAR(optimizer_switch = ... MySQL supports ...
https://dev.mysql.com/doc/refman/8.0/en/limit-optimization.html
In this case, as we can see from the EXPLAIN output, it uses the table's primary key. mysql> EXPLAIN SELECT c2 FROM t -> WHERE id2 > 3 -> ORDER BY id1 ASC LIMIT 2\G *************************** 1. If you need only a specified number of rows from a ...
https://dev.mysql.com/doc/refman/8.0/en/mysql-cluster-ndbd-definition.html
Changing the number of LDM threads normally requires a system restart to be effective and safe for cluster operations; this requirement is relaxed in certain cases, as explained later in this section. This can be overridden for data files, undo log ... The [ndbd] and [ndbd default] sections are used to configure the behavior of the cluster's data ...
https://dev.mysql.com/doc/refman/8.0/en/mysql-cluster-replication-conflict-resolution.html
Some types of conflict resolution supported by NDB Cluster (NDB$OLD(), NDB$MAX(), and NDB$MAX_DELETE_WIN(); additionally, in NDB 8.0.30 and later, NDB$MAX_INS() and NDB$MAX_DEL_WIN_INS()) implement this user-defined column as a “timestamp” ...