The EXPLAIN
statement provides
information about how MySQL executes statements:
EXPLAIN
works withSELECT
,DELETE
,INSERT
,REPLACE
, andUPDATE
statements.When
EXPLAIN
is 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 usingEXPLAIN
to obtain execution plan information, see Section 10.8.2, “EXPLAIN Output Format”.When
EXPLAIN
is used withFOR CONNECTION
rather than an explainable statement, it displays the execution plan for the statement executing in the named connection. See Section 10.8.4, “Obtaining Execution Plan Information for a Named Connection”.connection_id
For
SELECT
statements,EXPLAIN
produces additional execution plan information that can be displayed usingSHOW WARNINGS
. See Section 10.8.3, “Extended EXPLAIN Output Format”.EXPLAIN
is useful for examining queries involving partitioned tables. See Section 26.3.5, “Obtaining Information About Partitions”.The
FORMAT
option can be used to select the output format.TRADITIONAL
presents the output in tabular format. This is the default if noFORMAT
option is present.JSON
format 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 15.2.13, “SELECT Statement”.) However,
STRAIGHT_JOIN
may prevent indexes from being
used because it disables semijoin transformations. See
Section 10.2.2.1, “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
Section 10.15, “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 15.7.3.1, “ANALYZE TABLE Statement”.
EXPLAIN
can also be used to
obtain information about the columns in a table.
EXPLAIN
is synonymous
with tbl_name
DESCRIBE
and
tbl_name
SHOW COLUMNS FROM
. For more
information, see Section 15.8.1, “DESCRIBE Statement”, and
Section 15.7.7.5, “SHOW COLUMNS Statement”.
tbl_name