看到这个:Can you use index in mysql using "col1 OR col2"? 但我想我可以问这个相关的问题,以使讨论简单明了。
我有两个表:lists
和 agree_and_disagree_count
。
lists
表有两个 timestamp
字段 modified_on
和 first_publish_date
。这两个字段都有自己的索引。
agree_and_disagree_count
表还有一个 timestamp
字段 date_created
,该字段也已建立索引。
在同一个表中的字段的以下 OR 语句中,使用了两个索引。
mysql> EXPLAIN SELECT * FROM lists l
-> WHERE (l.modified_on > '2013-01-07 12:50:51' OR
-> l.first_publish_date > '2013-01-07 12:50:51')\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: l
type: index_merge
possible_keys: modifiedon,first_publish_date
key: modifiedon,first_publish_date
key_len: 4,9
ref: NULL
rows: 2
Extra: Using sort_union(modifiedon,first_publish_date); Using where
但是,如果字段来自不同的表,则看起来没有使用索引:
mysql> EXPLAIN SELECT * FROM lists l
-> JOIN agree_and_disagree_count adc ON adc.list_id=l.list_id
-> WHERE (l.modified_on > '2013-01-07 12:50:51' OR
-> adc.date_created > '2013-01-07 12:50:51')\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: adc
type: ALL
possible_keys: list_id_type_id,idx_list_id,idx_date_created
key: NULL
key_len: NULL
ref: NULL
rows: 5114907
Extra:
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: l
type: eq_ref
possible_keys: PRIMARY,modifiedon
key: PRIMARY
key_len: 4
ref: adc.list_id
rows: 1
Extra: Using where
为什么?
我不知道如何在评论中输入格式良好的回复,因此以下是对 @Iserni 的回复,他非常好心地帮助我:
我做了以下事情:
use test;
-- create tables
create table lists (
list_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
modified_on TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
first_publish_date DATETIME NULL DEFAULT NULL,
primary key (list_id)
) ENGINE=InnoDB;
create table agree_and_disagree_count (
id INT UNSIGNED NOT NULL AUTO_INCREMENT,
list_id int unsigned not null,
date_created TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
primary key (id)
) ENGINE=InnoDB;
-- create indices
create index index_modified_on on lists (modified_on);
create index index_first_publish_date on lists (first_publish_date);
create index index_data_created on agree_and_disagree_count (date_created);
-- load data
insert into lists (list_id, modified_on, first_publish_date) values
(1, now(), now()),
(2, now(), now()),
(3, now(), now());
insert into agree_and_disagree_count (list_id, date_created) values
(1, now()), (1, now()), (2, now());
-- query
EXPLAIN SELECT * FROM lists l JOIN agree_and_disagree_count adc
ON adc.list_id=l.list_id WHERE (l.modified_on > '2013-01-01 12:50:51'
OR adc.date_created > '2013-01-07 11:50:51');
+----+-------------+-------+------+----------------------------------+------+---------+------+------+--------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+----------------------------------+------+---------+------+------+--------------------------------+
| 1 | SIMPLE | l | ALL | PRIMARY,index_modified_on | NULL | NULL | NULL | 3 | |
| 1 | SIMPLE | adc | ALL | index_data_created,index_list_id | NULL | NULL | NULL | 3 | Using where; Using join buffer |
+----+-------------+-------+------+----------------------------------+------+---------+------+------+--------------------------------+
-- create new indexes suggested by Iserni
CREATE INDEX adc_test_ndx ON agree_and_disagree_count (list_id, date_created);
CREATE INDEX l_test_ndx ON lists (list_id, modified_on);
-- query
EXPLAIN SELECT * FROM lists l JOIN agree_and_disagree_count adc
ON adc.list_id=l.list_id WHERE (l.modified_on > '2013-01-01 12:50:51'
OR adc.date_created > '2013-01-07 11:50:51');
+----+-------------+-------+-------+-----------------------------------------------+--------------+---------+------+------+--------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+-----------------------------------------------+--------------+---------+------+------+--------------------------------+
| 1 | SIMPLE | adc | index | index_data_created,index_list_id,adc_test_ndx | adc_test_ndx | 8 | NULL | 3 | Using index |
| 1 | SIMPLE | l | ALL | PRIMARY,index_modified_on,l_test_ndx | NULL | NULL | NULL | 3 | Using where; Using join buffer |
+----+-------------+-------+-------+-----------------------------------------------+--------------+---------+------+------+--------------------------------+
看起来完整扫描仍在进行?
请查看 @Iserni 在接受的答案下方的评论。
最佳答案
一旦list_id
上的JOIN
开始,MySQL就无法将列表标识符与查询中使用并保存在索引中的字段“链接”,因为没有索引同时包含 ID 和这些字段;并且您无法对索引运行联接。
尝试创建这两个索引:
CREATE INDEX adc_test_ndx ON agree_and_disagree_count (list_id, date_created);
CREATE INDEX l_test_ndx ON lists (list_id, modified_on);
然后使用 EXPLAIN 重试。在我的系统上,我尝试创建两个最小的表,每个表有两条记录:
mysql> EXPLAIN SELECT * FROM lists l JOIN agree_and_disagree_count adc
ON adc.list_id=l.list_id WHERE (l.modified_on > '2013-01-01 12:50:51'
OR adc.date_created > '2013-01-07 11:50:51')\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: l
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 2
Extra:
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: adc
type: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 2
Extra: Using where; Using join buffer
2 rows in set (0.00 sec)
然后:
mysql> CREATE INDEX adc_test_ndx ON agree_and_disagree_count (list_id, date_created);
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> CREATE INDEX l_test_ndx ON lists (list_id, modified_on);
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> EXPLAIN SELECT * FROM lists l JOIN agree_and_disagree_count adc
ON adc.list_id=l.list_id WHERE (l.modified_on > '2013-01-01 12:50:51'
OR adc.date_created > '2013-01-07 11:50:51')\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: l
type: index
possible_keys: l_test_ndx
key: l_test_ndx
key_len: 9
ref: NULL
rows: 2
Extra: Using index
*************************** 2. row ***************************
id: 1
select_type: SIMPLE
table: adc
type: ref
possible_keys: adc_test_ndx
key: adc_test_ndx
key_len: 5
ref: test.l.list_id
rows: 1
Extra: Using where; Using index
2 rows in set (0.00 sec)
mysql>
正如预期的那样,我现在在两个表上都建立了索引。
你能发布表的结构和现有索引,或者更好的是,一个 SQLFiddle 吗?
关于当来自不同表的字段处于 OR 条件时,MySQL 不使用索引,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/14204651/