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Rechercher dans le manuel MySQL
8.8.1 Optimizing Queries with EXPLAIN
The EXPLAIN statement provides
information about how MySQL executes statements:
EXPLAINworks withSELECT,DELETE,INSERT,REPLACE, andUPDATEstatements.When
EXPLAINis used with an explainable statement, MySQL displays information from the optimizer about the statement execution plan. That is, MySQL explains how it would process the statement, including information about how tables are joined and in which order. For information about usingEXPLAINto obtain execution plan information, see Section 8.8.2, “EXPLAIN Output Format”.When
EXPLAINis used withFOR CONNECTIONrather than an explainable statement, it displays the execution plan for the statement executing in the named connection. See Section 8.8.4, “Obtaining Execution Plan Information for a Named Connection”.connection_idFor
SELECTstatements,EXPLAINproduces additional execution plan information that can be displayed usingSHOW WARNINGS. See Section 8.8.3, “Extended EXPLAIN Output Format”.EXPLAINis useful for examining queries involving partitioned tables. See Section 23.3.5, “Obtaining Information About Partitions”.The
FORMAToption can be used to select the output format.TRADITIONALpresents the output in tabular format. This is the default if noFORMAToption is present.JSONformat displays the information in JSON format.
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 13.2.10, “SELECT Syntax”.) However,
STRAIGHT_JOIN may prevent indexes from being
used because it disables semijoin transformations. See
Section 8.2.2.1, “Optimizing IN and EXISTS Subquery predicates with Semijoin
Transformations”.
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
MySQL
Internals: Tracing the Optimizer.
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 13.7.3.1, “ANALYZE TABLE Syntax”.
EXPLAIN can also be used to
obtain information about the columns in a table.
EXPLAIN
is synonymous
with tbl_nameDESCRIBE
and
tbl_nameSHOW COLUMNS FROM
. For more
information, see Section 13.8.1, “DESCRIBE Syntax”, and
Section 13.7.6.5, “SHOW COLUMNS Syntax”.
tbl_name
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Document créé le 26/06/2006, dernière modification le 26/10/2018
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