Spec-Zone .ru
спецификации, руководства, описания, API
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Spec-Zone .ru
спецификации, руководства, описания, API
|
The EXPLAIN statement can be used either as a way to obtain information about how
MySQL executes a statement, or as a synonym for DESCRIBE:
When you precede a statement with the keyword EXPLAIN, MySQL displays information from the optimizer about the query
execution plan. That is, MySQL explains how it would process the statement, including information about
how tables are joined and in which order. See Section 8.8.2,
"EXPLAIN Output Format".
EXPLAIN
EXTENDED can be used to obtain additional information; see Section
8.8.3, "EXPLAIN EXTENDED Output Format".
As of MySQL 5.6.3, EXPLAIN
provides information about SELECT,
DELETE,
INSERT,
REPLACE,
and UPDATE statements. Before MySQL 5.6.3, EXPLAIN provides information only about SELECT statements.
The following sections describe how to use EXPLAIN
and EXPLAIN
EXTENDED to obtain query execution plan information.
EXPLAIN PARTITIONS is useful only when examining queries involving
partitioned tables. For details, see Section
18.3.5, "Obtaining Information About Partitions".
EXPLAIN is synonymous
with tbl_nameDESCRIBE or tbl_nameSHOW COLUMNS FROM . For
information about tbl_nameDESCRIBE and SHOW COLUMNS, see Section 13.8.1,
"DESCRIBE Syntax", and Section
13.7.5.6, "SHOW COLUMNS Syntax".
With the help of EXPLAIN, you can see where you should add indexes to tables so that the statement
executes faster by using indexes to find rows. You can also use EXPLAIN to check whether the optimizer joins the tables in an optimal order.
To give a hint to the optimizer to use a join order corresponding to the order in which the tables are named in
a SELECT statement, begin the statement with SELECT
STRAIGHT_JOIN rather than just SELECT. (See Section 13.2.9, "SELECT Syntax".)
The optimizer trace may sometimes provide information complementary to that of EXPLAIN. However, the optimizer trace format and content are subject to change
between versions". For details, see .
If you have a problem with indexes not being used when you believe that they should be, run ANALYZE TABLE to update table statistics, such as cardinality of keys, that can
affect the choices the optimizer makes. See Section 13.7.2.1, "ANALYZE TABLE Syntax".