MySQL :: MySQL 5.7 Reference Manual :: 13.8.2 EXPLAIN Statement (original) (raw)

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 andEXPLAIN 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 theDESCRIBE andEXPLAIN 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 forSHOW COLUMNS. These statements also display information for views. The description forSHOW COLUMNS provides more information about the output columns. SeeSection 13.7.5.5, “SHOW COLUMNS Statement”.

By default, DESCRIBE displays information about all columns in the table.colname, 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, andSHOW 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 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 useEXPLAIN 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 aSELECT statement, begin the statement with SELECT STRAIGHT_JOIN rather than just SELECT. (SeeSection 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, seeSection 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. SeeSection 13.7.2.1, “ANALYZE TABLE Statement”.