相信很多人在mysql中看到了where條件中使用到了or就會以爲這樣是不會走索引的，通常會使用union all或者in 來進行優化，事實並不是想象的這樣具體問題具體分析。
下面我們來看看
首先我們用sysbench生成兩個100w行的表
表結構如下
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 specified1.首先我們使用同一列帶索引字段的進行查詢。
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 |
+----+-------------+---------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+從執行計劃中看出這樣是可以使用到索引的，另外我們使用in 或者union all來看。
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的執行計劃和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 |雖然執行計劃不通但union all估計的查詢行數和上面相同。
2.我們再來看看不同列帶索引字段的進行查詢
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 |
+----+-------------+---------+------------+-------------+---------------+---------+---------+------+------+----------+-----------------------------------這樣的情況也會使用索引
如果or的條件中有個條件不帶索引的話，那這條sql就不會使用到索引了，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列沒索引所以整條的sql就不會使用到索引
假設使用union all來改寫一樣需要全表掃描所以意義也不大，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.接下來我們看看多表關聯查詢
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> 可以看出在多表查詢的情況下or條件如果不在同一個表內執行計劃表a的查詢不走索引。
我們試試看用union all來進行改寫
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的條件不在同一個表的情況下 使用union all來改寫掃描行數減少且會走索引。
https://www.cnblogs.com/soysauce/p/10414296.html