MySQL 5.1 英文手册 :: 12.8.2 EXPLAIN Syntax

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MySQL 5.1 Reference Manual :: 12 SQL Statement Syntax :: 12.8 MySQL Utility Statements :: 12.8.2 EXPLAIN Syntax
12.8.2. EXPLAIN Syntax
```
EXPLAIN tbl_name
```
Or:
```
EXPLAIN [EXTENDED | PARTITIONS] SELECT select_options
```
The EXPLAIN statement can be used either as a synonym for DESCRIBE or as a way to obtain information about how MySQL executes a SELECT statement:
- EXPLAIN tbl_name is synonymous with DESCRIBE tbl_name or SHOW COLUMNS FROM tbl_name.
  
  For a description of the DESCRIBE and SHOW COLUMNS statements, see Section 12.8.1, “DESCRIBE Syntax”, and Section 12.4.5.6, “SHOW COLUMNS Syntax”.
- When you precede a SELECT statement with the keyword EXPLAIN, MySQL displays information from the optimizer about the query execution plan. That is, MySQL explains how it would process the SELECT, including information about how tables are joined and in which order. EXPLAIN EXTENDED can be used to provide additional information.
  
  For information regarding the use of EXPLAIN and EXPLAIN EXTENDED for obtaining query execution plan information, see Section 7.2.1, “Optimizing Queries with EXPLAIN”.
- EXPLAIN PARTITIONS is available beginning with MySQL 5.1.5. It is useful only when examining queries involving partitioned tables.
  
  For details, see Section 18.3.4, “Obtaining Information About Partitions”.