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.

• Obtaining Table Structure Information

• Obtaining Execution Plan Information

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 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”.

Obtaining Execution Plan Information

The EXPLAIN statement provides information about how MySQL executes statements:

• EXPLAIN works with SELECT, DELETE, INSERT, REPLACE, and UPDATE 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 using EXPLAIN to obtain execution plan information, see Section 8.8.2, “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 8.8.4, “Obtaining Execution Plan Information for a Named Connection”.

• For SELECT statements, EXPLAIN produces additional execution plan information that can be displayed using SHOW WARNINGS. See Section 8.8.3, “Extended EXPLAIN Output Format”.

  Note

  In 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 the EXTENDED keyword is superfluous and deprecated. Its use results in a warning, and it is removed from EXPLAIN syntax in MySQL 8.0.

• EXPLAIN is useful for examining queries involving partitioned tables. See Section 22.3.5, “Obtaining Information About Partitions”.

  Note

  In 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 the PARTITIONS keyword is superfluous and deprecated. Its use results in a warning, and it is removed from EXPLAIN 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 no FORMAT option is present. JSON format displays the information in JSON format.

  For complex statements, the JSON output can be quite large; in particular, it can be difficult when reading it to pair the closing bracket and opening brackets; to cause the JSON structure's key, if it has one, to be repeated near the closing bracket, set end_markers_in_json=ON. You should be aware that while this makes the output easier to read, it also renders the JSON invalid, causing JSON functions to raise an error.

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 Section 8.15, “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 13.7.2.1, “ANALYZE TABLE Statement”.

Note

MySQL Workbench has a Visual Explain capability that provides a visual representation of EXPLAIN output. See Tutorial: Using Explain to Improve Query Performance.