一個包含查詢所需的字段的索引稱為 covering index 覆寫索引。MySQL隻需要通過索引就可以傳回查詢所需要的資料,而不必在查到索引之後進行回表操作,減少IO,提供效率。
當你對一個sql 使用explain statement 檢視一個sql的執行計劃時,在EXPLAIN的Extra列出現Using Index提示時,就說明該select查詢使用了覆寫索引。
【使用場景】
生産過程中遇到的一個例子,且先不讨論 count(字段)還是 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;
+----+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
| 1 | SIMPLE | o | index_merge | buyer_id,order_status | buyer_id,order_status | 9,1 | NULL | 3852 | Using intersect(buyer_id,order_status); Using where |
1 row in set (0.00 sec)
上述select語句的執行計劃使用了index_merge 索引聚合,整體sql執行花費0.22s 。根據查詢條件對表結構進行如下調整:
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;
+----+-------------+-------+------+----------------------+----------------------+---------+-------------------+-------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
| 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 |
執行計劃使用了 using index --覆寫索引而且執行時間由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;
【覆寫索引的限制】
遇到以下情況,執行計劃不會選擇覆寫查詢:
1select選擇的字段中含有不在索引中的字段 ,也即索引沒有覆寫全部的列。
2 where 條件中不能含有對索引進行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 ---覆寫索引掃描
Extra: Using where; Using index