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https://dev.mysql.com/doc/refman/8.0/en/internal-temporary-tables.html
To determine whether a statement requires a temporary table, use EXPLAIN and check the Extra column to see whether it says Using temporary (see Section 10.8.1, “Optimizing Queries with EXPLAIN”). EXPLAIN does not necessarily say Using temporary ... In some cases, the server creates internal temporary tables while processing ...
https://dev.mysql.com/doc/refman/8.0/en/optimizer-issues.html
For the cases when MySQL does not do the "right" thing, tools that you have available to help MySQL are: Use the EXPLAIN statement to get information about how MySQL processes a query. To use it, just add the keyword EXPLAIN to the front of your ...
https://dev.mysql.com/doc/refman/8.0/en/range-optimization.html
The key_len column in the EXPLAIN output indicates the maximum length of the key prefix used. For EXPLAIN FOR CONNECTION, the output changes as follows if index dives are skipped: For traditional output, the rows and filtered values are NULL. The ...
https://dev.mysql.com/doc/refman/8.0/en/bug-reports.html
The EXPLAIN SELECT example later in this section demonstrates the use of \G. Before posting a bug report about a problem, please try to verify that it is a bug and that it has not been reported already: Start by searching the MySQL online manual at ...
https://dev.mysql.com/doc/refman/8.0/en/condition-filtering.html
In EXPLAIN output, the rows column indicates the row estimate for the chosen access method, and the filtered column reflects the effect of condition filtering. An exception occurs for EXPLAIN: To provide more information, the filtering effect is ...
https://dev.mysql.com/doc/refman/8.0/en/derived-table-optimization.html
Consider the following EXPLAIN statement, for a SELECT query that contains a derived table: EXPLAIN SELECT * FROM (SELECT * FROM t1) AS derived_t1; The optimizer avoids materializing the derived table by delaying it until the result is needed during ...The optimizer handles derived tables, view references, and common table expressions the same way: It avoids unnecessary materialization whenever possible, which enables pushing down conditions from the outer query to derived tables and produces more efficient execution ...
https://dev.mysql.com/doc/refman/8.0/en/information-schema-optimization.html
To also see the query used by the server to execute an INFORMATION_SCHEMA query, use SHOW WARNINGS immediately following EXPLAIN. How does the server process that statement? To find out, use EXPLAIN: mysql> EXPLAIN SELECT COLLATION_NAME FROM ...
https://dev.mysql.com/doc/refman/8.0/en/mysql-cluster-options-variables.html
Note Joins tested using EXPLAIN that can be pushed down contribute to this number. Note Executing EXPLAIN on joins that can be pushed down to NDB does not add to this number. This section provides information about MySQL server options, server and ...
https://dev.mysql.com/doc/refman/8.0/en/select-optimization.html
You can use the EXPLAIN statement to determine which indexes are used for a SELECT. See Section 10.3.1, “How MySQL Uses Indexes” and Section 10.8.1, “Optimizing Queries with EXPLAIN”. If a performance issue is not easily solved by one of the ... Queries, in the form of SELECT statements, perform all the lookup operations in the ...
https://dev.mysql.com/doc/refman/8.0/en/semijoins.html
EXPLAIN output indicates the use of semijoin strategies as follows: For extended EXPLAIN output, the text displayed by a following SHOW WARNINGS shows the rewritten query, which displays the semijoin structure. (See Section 10.8.3, “Extended ... A ...