强制MySQL在Join上使用两个索引

Question

我试图强迫MySQL使用两个索引。我正在加入一个表，我想利用两个索引之间的交叉。具体术语是“使用相交”，这是指向MySQL文档的链接：

http://dev.mysql.com/doc/refman/5.0/zh-CN/index-merge-
optimization.html

有什么办法可以强制实施吗？我的查询正在使用它（并且它加快了速度），但是现在无论出于何种原因它都停止了。

这是我要继续执行的JOIN。我希望查询使用的两个索引是scs.CONSUMER_ID_1和scs_CONSUMER_ID_2

JOIN survey_customer_similarity AS scs
    ON cr.CONSUMER_ID=scs.CONSUMER_ID_2 
    AND cal.SENDER_CONSUMER_ID=scs.CONSUMER_ID_1 
    OR cr.CONSUMER_ID=scs.CONSUMER_ID_1 
    AND cal.SENDER_CONSUMER_ID=scs.CONSUMER_ID_2

Answer 1

参见MySQL文档FORCE INDEX。

JOIN survey_customer_similarity AS scs 
FORCE INDEX (CONSUMER_ID_1,CONSUMER_ID_2)
ON
cr.CONSUMER_ID=scs.CONSUMER_ID_2 
AND cal.SENDER_CONSUMER_ID=scs.CONSUMER_ID_1 
OR cr.CONSUMER_ID=scs.CONSUMER_ID_1 
AND cal.SENDER_CONSUMER_ID=scs.CONSUMER_ID_2

正如TheScrumMeister在下面指出的那样，它是否可以同时实际使用两个索引取决于您的数据。

这是一个示例，您需要强制该表出现两次以控制查询的执行和交集。

使用它来创建一个包含100K条记录的表，其中约1K行与过滤条件匹配，i in (2,3)而1K行与j in (2,3)以下条件匹配：

drop table if exists t1;
create table t1 (id int auto_increment primary key, i int, j int);
create index ix_t1_on_i on t1(i);
create index ix_t1_on_j on t1(j);
insert into t1 (i,j) values (2,2),(2,3),(4,5),(6,6),(2,6),(2,7),(3,2);
insert into t1 (i,j) select i*2, j*2+i from t1;
insert into t1 (i,j) select i*2, j*2+i from t1;
insert into t1 (i,j) select i*2, j*2+i from t1;
insert into t1 (i,j) select i*2, j*2+i from t1;
insert into t1 (i,j) select i*2, j*2+i from t1;
insert into t1 (i,j) select i*2, j*2+i from t1;
insert into t1 (i,j) select i*2, j*2+i from t1;
insert into t1 (i,j) select i*2, j*2+i from t1;
insert into t1 (i,j) select i*2, j*2+i from t1;
insert into t1 (i,j) select i*2, j*2+i from t1;
insert into t1 (i,j) select i*2, j*2+i from t1;
insert into t1 (i,j) select i*2, j*2+i from t1;
insert into t1 (i,j) select i, j from t1;
insert into t1 (i,j) select i, j from t1;
insert into t1 (i,j) select 2, j from t1 where not j in (2,3) limit 1000;
insert into t1 (i,j) select i, 3 from t1 where not i in (2,3) limit 1000;

进行时：

select t.* from t1 as t where t.i=2 and t.j=3 or t.i=3 and t.j=2

您将获得8场匹配的比赛：

+-------+------+------+
| id    | i    | j    |
+-------+------+------+
|     7 |    3 |    2 |
| 28679 |    3 |    2 |
| 57351 |    3 |    2 |
| 86023 |    3 |    2 |
|     2 |    2 |    3 |
| 28674 |    2 |    3 |
| 57346 |    2 |    3 |
| 86018 |    2 |    3 |
+-------+------+------+

EXPLAIN在上面的查询中使用以获取：

id | select_type | table | type  | possible_keys         | key        | key_len | ref  | rows | Extra
1  | SIMPLE      | t     | range | ix_t1_on_i,ix_t1_on_j | ix_t1_on_j | 5       | NULL | 1012 | Using where

即使我们FORCE INDEX在两个索引上添加查询，EXPLAIN也将返回 完全相同的内容 。

要使其跨两个索引收集，然后相交，请使用以下命令：

select t.* from t1 as a force index(ix_t1_on_i)

join t1 as b force index(ix_t1_on_j) on a.id=b.id

where a.i=2 and b.j=3 or a.i=3 and b.j=2

使用该查询explain可获取：

id | select_type | table | type  | possible_keys | key        | key_len | ref  | rows | Extra
1  | SIMPLE      | a     | range | ix_t1_on_i    | ix_t1_on_i | 5       | NULL | 1019 | Using where
1  | SIMPLE      | b     | range | ix_t1_on_j    | ix_t1_on_j | 5       | NULL | 1012 | Using where; Using index

这证明正在使用索引。但这可能会更快，也可能不会更快，这取决于许多其他因素。