13.8.2 EXPLAIN Statement
{EXPLAIN | DESCRIBE | DESC}
tbl_name [col_name | wild]
{EXPLAIN | DESCRIBE | DESC}
[explain_type]
{explainable_stmt | FOR CONNECTION connection_id}
explain_type: {
EXTENDED
| PARTITIONS
| FORMAT = format_name
}
format_name: {
TRADITIONAL
| JSON
}
explainable_stmt: {
SELECT statement
| DELETE statement
| INSERT statement
| REPLACE statement
| UPDATE statement
}
The DESCRIBE
and EXPLAIN
statements are synonyms. In practice, the DESCRIBE
keyword is more often used to obtain information about table structure, whereas EXPLAIN
is used to obtain a query execution plan (that is, an explanation of how MySQL would execute a query).
The following discussion uses the DESCRIBE
and EXPLAIN
keywords in accordance with those uses, but the MySQL parser treats them as completely synonymous.
DESCRIBE
provides information about the columns in a table:
mysql> DESCRIBE City;
+------------+----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+----------+------+-----+---------+----------------+
| Id | int(11) | NO | PRI | NULL | auto_increment |
| Name | char(35) | NO | | | |
| Country | char(3) | NO | UNI | | |
| District | char(20) | YES | MUL | | |
| Population | int(11) | NO | | 0 | |
+------------+----------+------+-----+---------+----------------+
DESCRIBE
is a shortcut for SHOW COLUMNS
. These statements also display information for views. The description for SHOW COLUMNS
provides more information about the output columns. See Section 13.7.5.5, “SHOW COLUMNS Statement”.
By default, DESCRIBE
displays information about all columns in the table. col_name
, if given, is the name of a column in the table. In this case, the statement displays information only for the named column. wild
, if given, is a pattern string. It can contain the SQL %
and _
wildcard characters. In this case, the statement displays output only for the columns with names matching the string. There is no need to enclose the string within quotation marks unless it contains spaces or other special characters.
The DESCRIBE
statement is provided for compatibility with Oracle.
The SHOW CREATE TABLE
, SHOW TABLE STATUS
, and SHOW INDEX
statements also provide information about tables. See Section 13.7.5, “SHOW Statements”.
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 8.8.2, “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 8.8.4, “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 8.8.3, “Extended EXPLAIN Output Format”.NoteIn older MySQL releases, extended information was produced using
EXPLAIN EXTENDED
. That syntax is still recognized for backward compatibility but extended output is now enabled by default, so theEXTENDED
keyword is superfluous and deprecated. Its use results in a warning, and it is removed fromEXPLAIN
syntax in MySQL 8.0.EXPLAIN
is useful for examining queries involving partitioned tables. See Section 22.3.5, “Obtaining Information About Partitions”.NoteIn older MySQL releases, partition information was produced using
EXPLAIN PARTITIONS
. That syntax is still recognized for backward compatibility but partition output is now enabled by default, so thePARTITIONS
keyword is superfluous and deprecated. Its use results in a warning, and it is removed fromEXPLAIN
syntax in MySQL 8.0.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.
EXPLAIN
requires the same privileges required to execute the explained statement. Additionally, EXPLAIN
also requires the SHOW VIEW
privilege for any explained view.
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 Statement”.)
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 13.7.2.1, “ANALYZE TABLE Statement”.
MySQL Workbench has a Visual Explain capability that provides a visual representation of EXPLAIN
output. See Tutorial: Using Explain to Improve Query Performance .