相信很多人在MySQL中看到了where条件中使用到了or就会以为这样是不会走索引的，通常会使用union all或者in 来进行优化，事实并不是想象的这样具体问题具体分析。
下面我们来看看
首先我们用sysbench生成两个100w行的表
表结构如下
mysql> show create table sbtest1 G; *************************** 1. row *************************** Table: sbtest1 Create Table: CREATE TABLE `sbtest1` ( `id` int(11) NOT NULL AUTO_INCREMENT, `k` int(11) NOT NULL DEFAULT '0', `c` char(120) NOT NULL DEFAULT '', `pad` char(60) NOT NULL DEFAULT '', PRIMARY KEY (`id`), KEY `k_1` (`k`), KEY `c_1` (`c`) ) ENGINE=InnoDB AUTO_INCREMENT=1000001 DEFAULT CHARSET=latin1 1 row in set (0.00 sec)
ERROR: No query specified
mysql> show create table sbtest2 G; *************************** 1. row *************************** Table: sbtest2 Create Table: CREATE TABLE `sbtest2` ( `id` int(11) NOT NULL AUTO_INCREMENT, `k` int(11) NOT NULL DEFAULT '0', `c` char(120) NOT NULL DEFAULT '', `pad` char(60) NOT NULL DEFAULT '', PRIMARY KEY (`id`), KEY `k_2` (`k`), KEY `c_2` (`c`) ) ENGINE=InnoDB AUTO_INCREMENT=1000001 DEFAULT CHARSET=latin1 1 row in set (0.00 sec)
ERROR: No query specified
1.首先我们使用同一列带索引字段的进行查询。
mysql> explain select * from sbtest1 where k='501462' or k='502480'; ---- ------------- --------- ------------ ------- --------------- ------ --------- ------ ------ ---------- ----------------------- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | ---- ------------- --------- ------------ ------- --------------- ------ --------- ------ ------ ---------- ----------------------- | 1 | SIMPLE | sbtest1 | NULL | range | k_1 | k_1 | 4 | NULL | 214 | 100.00 | Using index condition | ---- ------------- --------- ------------ ------- --------------- ------ --------- ------ ------ ---------- -----------------------
从执行计划中看出这样是可以使用到索引的，另外我们使用in 或者union all来看。
mysql> explain select pad from sbtest1 where k in ('501462','502480'); ---- ------------- --------- ------------ ------- --------------- ------ --------- ------ ------ ---------- ----------------------- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | ---- ------------- --------- ------------ ------- --------------- ------ --------- ------ ------ ---------- ----------------------- | 1 | SIMPLE | sbtest1 | NULL | range | k_1 | k_1 | 4 | NULL | 214 | 100.00 | Using index condition | ---- ------------- --------- ------------ ------- --------------- ------ --------- ------ ------ ---------- -----------------------
in的执行计划和or相同。
mysql> explain select pad from sbtest1 where k='501462' union all select pad from sbtest1 where k='502480'; ---- ------------- --------- ------------ ------ --------------- ------ --------- ------- ------ ---------- ------- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | ---- ------------- --------- ------------ ------ --------------- ------ --------- ------- ------ ---------- ------- | 1 | PRIMARY | sbtest1 | NULL | ref | k_1 | k_1 | 4 | const | 113 | 100.00 | NULL | | 2 | UNION | sbtest1 | NULL | ref | k_1 | k_1 | 4 | const | 101 | 100.00 | NULL |
虽然执行计划不通但union all估计的查询行数和上面相同。
2.我们再来看看不同列带索引字段的进行查询
mysql> explain select pad from sbtest1 where k='501462' or c='68487932199-96439406143-93774651418-41631865787-96406072701-20604855487-25459966574-28203206787-41238978918-19503783441'; ---- ------------- --------- ------------ ------------- --------------- --------- --------- ------ ------ ---------- ----------------------------------- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | ---- ------------- --------- ------------ ------------- --------------- --------- --------- ------ ------ ---------- ----------------------------------- | 1 | SIMPLE | sbtest1 | NULL | index_merge | k_1,c_1 | k_1,c_1 | 4,120 | NULL | 114 | 100.00 | Using union(k_1,c_1); Using where | ---- ------------- --------- ------------ ------------- --------------- --------- --------- ------ ------ ---------- -----------------------------------
这样的情况也会使用索引
如果or的条件中有个条件不带索引的话，那这条sql就不会使用到索引了，如下。
mysql> explain select pad from sbtest1 where k='501462' or pad='00592560354-80393027097-78244247549-39135306455-88936868384'; ---- ------------- --------- ------------ ------ --------------- ------ --------- ------ -------- ---------- ------------- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | ---- ------------- --------- ------------ ------ --------------- ------ --------- ------ -------- ---------- ------------- | 1 | SIMPLE | sbtest1 | NULL | ALL | k_1 | NULL | NULL | NULL | 986400 | 19.00 | Using where | ---- ------------- --------- ------------ ------ --------------- ------ --------- ------ -------- ---------- -------------
pad列没索引所以整条的sql就不会使用到索引
假设使用union all来改写一样需要全表扫描所以意义也不大，如下
mysql> explain select pad from sbtest1 where k='501462' union all select pad from sbtest1 where pad='00592560354-80393027097-78244247549-39135306455-88936868384'; ---- ------------- --------- ------------ ------ --------------- ------ --------- ------- -------- ---------- ------------- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | ---- ------------- --------- ------------ ------ --------------- ------ --------- ------- -------- ---------- ------------- | 1 | PRIMARY | sbtest1 | NULL | ref | k_1 | k_1 | 4 | const | 113 | 100.00 | NULL | | 2 | UNION | sbtest1 | NULL | ALL | NULL | NULL | NULL | NULL | 986400 | 10.00 | Using where | ---- ------------- --------- ------------ ------ --------------- ------ --------- ------- -------- ---------- -------------
3.接下来我们看看多表关联查询
mysql> explain select a.pad,b.pad from sbtest1 a,sbtest2 b where a.id=b.id and (a.c='123' or b.c='1234'); ---- ------------- ------- ------------ -------- --------------- --------- --------- ----------- -------- ---------- ------------- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | ---- ------------- ------- ------------ -------- --------------- --------- --------- ----------- -------- ---------- ------------- | 1 | SIMPLE | a | NULL | ALL | PRIMARY,c_1 | NULL | NULL | NULL | 986400 | 100.00 | NULL | | 1 | SIMPLE | b | NULL | eq_ref | PRIMARY,c_2 | PRIMARY | 4 | test.a.id | 1 | 100.00 | Using where | ---- ------------- ------- ------------ -------- --------------- --------- --------- ----------- -------- ---------- ------------- 2 rows in set, 1 warning (0.00 sec)
mysql> explain select a.pad,b.pad from sbtest1 a,sbtest2 b where a.id=b.id and (a.c='123' or a.c='1234'); ---- ------------- ------- ------------ -------- --------------- --------- --------- ----------- ------ ---------- ----------------------- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | ---- ------------- ------- ------------ -------- --------------- --------- --------- ----------- ------ ---------- ----------------------- | 1 | SIMPLE | a | NULL | range | PRIMARY,c_1 | c_1 | 120 | NULL | 2 | 100.00 | Using index condition | | 1 | SIMPLE | b | NULL | eq_ref | PRIMARY | PRIMARY | 4 | test.a.id | 1 | 100.00 | NULL | ---- ------------- ------- ------------ -------- --------------- --------- --------- ----------- ------ ---------- ----------------------- 2 rows in set, 1 warning (0.00 sec)
mysql>
可以看出在多表查询的情况下or条件如果不在同一个表内执行计划表a的查询不走索引。
我们试试看用union all来进行改写
mysql> explain select a.pad,a.c from sbtest1 a,sbtest2 b where a.id=b.id and a.c='123' union all select a.pad,a.c from sbtest1 a,sbtest2 b where a.id=b.id and b.c='1234'; ---- ------------- ------- ------------ -------- --------------- --------- --------- ----------- ------ ---------- ------------- | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | ---- ------------- ------- ------------ -------- --------------- --------- --------- ----------- ------ ---------- ------------- | 1 | PRIMARY | a | NULL | ref | PRIMARY,c_1 | c_1 | 120 | const | 1 | 100.00 | NULL | | 1 | PRIMARY | b | NULL | eq_ref | PRIMARY | PRIMARY | 4 | test.a.id | 1 | 100.00 | Using index | | 2 | UNION | b | NULL | ref | PRIMARY,c_2 | c_2 | 120 | const | 1 | 100.00 | Using index | | 2 | UNION | a | NULL | eq_ref | PRIMARY | PRIMARY | 4 | test.b.id | 1 | 100.00 | NULL | ---- ------------- ------- ------------ -------- --------------- --------- --------- ----------- ------ ---------- -------------
在or的条件不在同一个表的情况下 使用union all来改写扫描行数减少且会走索引。
