MySQL explain
Explain
Explain用来查看执行计划。可用于SELECT, DELETE, INSERT, REPLACE, and UPDATE statements
例子
mysql> explain SELECT COUNT(*) FROM vehicle_prod WHERE fin LIKE '%00000' \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: vehicle_prod
partitions: NULL
type: index
possible_keys: NULL
key: idx_vehicle_prod_fin
key_len: 203
ref: NULL
rows: 3182812
filtered: 11.11
Extra: Using where; Using index
1 row in set, 1 warning (0.00 sec)
其中：
- id
- The SELECT identifier
- select_type
- The SELECT type
- SIMPLE:Simple SELECT (not using UNION or subqueries)
- PRIMARY:Outermost SELECT
- UNION:Second or later SELECT statement in a UNION
- SUBQUERY:First SELECT in subquery
- ...
- table
- The table for the output row
- partitions
- The matching partitions
- type
- The join type
- possible_keys
- The possible indexes to choose
- key
- The index actually chosen
- key_len
- The length of the chosen key
- ref
- The columns compared to the index
- rows
- Estimate of rows to be examined
- filtered
- Percentage of rows filtered by table condition
- Extra
- Additional information
It is possible for key to name an index that is not present in the possible_keys value. This can happen if none of the possible_keys indexes are suitable for looking up rows, but all the columns selected by the query are columns of some other index. That is, the named index covers the selected columns, so although it is not used to determine which rows to retrieve, an index scan is more efficient than a data row scan.
Extra显示的索引扫描方式:
- using where：查询使用索引的情况下，需要回表去查询所需的数据。
- using index condition：查询使用了索引，但是需要回表查询数据。
- using index：查询使用覆盖索引的时候会出现。
- using index & using where：查询使用了索引，但是需要的数据都在索引列中能找到，不需要回表查询数据。