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MySQL 5.7 Reference Manual
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Excerpts from this Manual

14.8.2 EXPLAIN Syntax

    tbl_name [col_name | wild]

    {explainable_stmt | FOR CONNECTION connection_id}

explain_type: {
  | FORMAT = format_name

format_name: {
  | 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.

Obtaining Table Structure Information

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, “SHOW COLUMNS Syntax”.

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 14.7.5, “SHOW Syntax”.

Obtaining Execution Plan Information

The EXPLAIN statement provides information about how MySQL executes statements:

  • In MySQL 5.7, permitted explainable statements for EXPLAIN are SELECT, DELETE, INSERT, REPLACE, and UPDATE.

  • 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 using EXPLAIN to obtain execution plan information, see Section 9.8.2, “EXPLAIN Output Format”.

  • When EXPLAIN is used with FOR CONNECTION connection_id rather than an explainable statement, it displays the execution plan for the statement executing in the named connection. See Section 9.8.4, “Obtaining Execution Plan Information for a Named Connection”.

  • EXPLAIN EXTENDED can be used to obtain additional execution plan information. See Section 9.8.3, “EXPLAIN EXTENDED Output Format”.

    As of MySQL 5.7.3, the EXPLAIN statement is changed so that the effect of the EXTENDED keyword is always enabled. EXTENDED is still recognized for backward compatibility, but is superfluous and is deprecated; its use results in a warning. It will be removed from EXPLAIN syntax in a future MySQL release.

  • EXPLAIN PARTITIONS is useful for examining queries involving partitioned tables. See Section 21.3.5, “Obtaining Information About Partitions”.

    As of MySQL 5.7.3, the EXPLAIN statement is changed so that the effect of the PARTITIONS keyword is always enabled. PARTITIONS is still recognized for backward compatibility, but is superfluous and is deprecated; its use results in a warning. It will be removed from EXPLAIN syntax in a future MySQL release.

  • The FORMAT option can be used to select the output format. TRADITIONAL presents the output in tabular format. This is the default if no FORMAT option is present. JSON format displays the information in JSON format. With FORMAT = JSON, the output includes extended and partition information.

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 14.2.9, “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 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, “ANALYZE TABLE Syntax”.

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