【MySQL】性能优化之覆盖索引-白红宇

一个包含查询所需的字段的索引称为 covering index 覆盖索引。MySQL只需要通过索引就可以返回查询所需要的数据，而不必在查到索引之后进行回表操作，减少IO，提供效率。

当你对一个sql 使用explain statement 查看一个sql的执行计划时，在EXPLAIN的Extra列出现Using Index提示时，就说明该select查询使用了覆盖索引。

【使用场景】

生产过程中遇到的一个例子，且先不讨论 count(字段)还是 count(*)

root@yang 06:38:34>select count(o.order_id) as cnt from `order` o WHERE o.settle_type = 2 and o.order_status = 2 and o.buyer_id=1979459339672858;

+------+

| cnt |

+------+

| 7574 |

+------+

1 row in set (0.22 sec)

root@yang 06:36:38>explain select count(o.order_id) as cnt from `order` o WHERE o.settle_type = 2 and o.order_status = 2 and o.buyer_id=1979459339672858;

+----+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-----------------------------------------------------+

+----+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-----------------------------------------------------+

+----+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-----------------------------------------------------+

1 row in set (0.00 sec)

上述select语句的执行计划使用了index_merge 索引聚合，整体sql执行花费0.22s 。根据查询条件对表结构进行如下调整：

root@yang 06:33:14>alter table `ordert` add key ind_od_bid_st_ty_oid(`buyer_id`,`order_status`,`settle_type`,`order_id`);

Query OK, 0 rows affected (3.00 sec)

Records: 0 Duplicates: 0 Warnings: 0

root@yang 06:38:50>explain select count(o.order_id) as cnt from `ordert` o WHERE o.settle_type = 2 and o.order_status = 2 and o.buyer_id=1979459339672858;

+----+-------------+-------+------+----------------------+----------------------+---------+-------------------+-------+--------------------------+

+----+-------------+-------+------+----------------------+----------------------+---------+-------------------+-------+--------------------------+

| 1 | SIMPLE | o | ref | ind_od_bid_st_ty_oid | ind_od_bid_st_ty_oid | 11 | const,const,const | 15242 | Using where; Using index |

+----+-------------+-------+------+----------------------+----------------------+---------+-------------------+-------+--------------------------+

1 row in set (0.00 sec)

执行计划使用了 using index --覆盖索引而且执行时间由0.22s，下降到小于0.01s。

root@yang 06:39:06>select count(o.order_id) as cnt from `ordert` o WHERE o.settle_type = 2 and o.order_status = 2 and o.buyer_id=1979459339672858;

+------+

| cnt |

+------+

| 7574 |

+------+

1 row in set (0.00 sec)

【覆盖索引的限制】

遇到以下情况，执行计划不会选择覆盖查询：

1select选择的字段中含有不在索引中的字段，也即索引没有覆盖全部的列。

2 where 条件中不能含有对索引进行like的操作。

root@odbsyunying 08:18:15>explain select count(*) as cnt from `ordert` o WHERE o.settle_type = 2

> and o.order_status = 2

> and o.buyer_id like '1979459339672858' \G

*************************** 1. row ***************************

id: 1

select_type: SIMPLE

table: o

type: index

possible_keys: ind_od_bid_st_ty_oid

key: ind_od_bid_st_ty_oid

key_len: 19

ref: NULL

rows: 767466 ---覆盖索引扫描

Extra: Using where; Using index

1 row in set (0.00 sec)