为什么MySQL不在WHERE IF子句中使用索引？

Question

由于这个设置:

mysql> show global variables like '%indexes';
+-------------------------------+-------+
| Variable_name                 | Value |
+-------------------------------+-------+
| log_queries_not_using_indexes | ON    | 
+-------------------------------+-------+

慢查询日志继续接收:

# Time: 120607 16:58:30
# User@Host: xbtit[xbtit] @  [123.30.53.244]
# Query_time: 0  Lock_time: 0  Rows_sent: 1  Rows_examined: 16006
SELECT * FROM xbtit_files WHERE IF(soha_id is null OR soha_id = '', info_hash, soha_id)='6d63dd4ab199190b531752067414d4d6e6568f90';

试图解释这个查询:

mysql> EXPLAIN SELECT * FROM xbtit_files WHERE IF(soha_id is null OR soha_id = '', info_hash, soha_id)='6d63dd4ab199190b531752067414d4d6e6568f90';
+----+-------------+-------------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table       | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | xbtit_files | ALL  | NULL          | NULL | NULL    | NULL | 16006 | Using where | 
+----+-------------+-------------+------+---------------+------+---------+------+-------+-------------+

让我感到惊讶的是MySQL不使用索引的原因:

mysql> show index from xbtit_files;
+-------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table       | Non_unique | Key_name  | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| xbtit_files |          0 | PRIMARY   |            1 | info_hash   | A         |       16006 |     NULL | NULL   |      | BTREE      |         | 
| xbtit_files |          1 | filename  |            1 | filename    | A         |       16006 |     NULL | NULL   | YES  | BTREE      |         | 
| xbtit_files |          1 | category  |            1 | category    | A         |           1 |     NULL | NULL   |      | BTREE      |         | 
| xbtit_files |          1 | uploader  |            1 | uploader    | A         |          16 |     NULL | NULL   |      | BTREE      |         | 
| xbtit_files |          1 | bin_hash  |            1 | bin_hash    | A         |       16006 |       20 | NULL   |      | BTREE      |         | 
| xbtit_files |          1 | ix_sohaid |            1 | soha_id     | A         |       16006 |     NULL | NULL   | YES  | BTREE      |         | 
+-------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+

FORCE INDEX 也不起作用:

mysql> EXPLAIN SELECT * FROM xbtit_files force index (PRIMARY) WHERE IF(soha_id is null OR soha_id = '', info_hash, soha_id)='6d63dd4ab199190b531752067414d4d6e6568f90';
+----+-------------+-------------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table       | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | xbtit_files | ALL  | NULL          | NULL | NULL    | NULL | 16006 | Using where | 
+----+-------------+-------------+------+---------------+------+---------+------+-------+-------------+

我必须将此查询拆分为2个操作吗？

Answer 1

在 中MySQL，您无法在表达式上创建索引，并且优化器不够智能，无法根据两个索引拆分查询。

用这个：

SELECT  *
FROM    xbtit_files 
WHERE   soha_id = '6d63dd4ab199190b531752067414d4d6e6568f90'
UNION ALL
SELECT  *
FROM    xbtit_files 
WHERE   soha_id = ''
        AND info_hash = '6d63dd4ab199190b531752067414d4d6e6568f90'
UNION ALL
SELECT  *
FROM    xbtit_files 
WHERE   soha_id IS NULL
        AND info_hash = '6d63dd4ab199190b531752067414d4d6e6568f90'

每个查询都使用自己的索引。

您可以将其组合成一个查询：

SELECT  *
FROM    xbtit_files 
WHERE   (
        soha_id = '6d63dd4ab199190b531752067414d4d6e6568f90'
        OR
        (soha_id = '' AND info_hash = '6d63dd4ab199190b531752067414d4d6e6568f90')
        OR
        (soha_id IS NULL AND info_hash = '6d63dd4ab199190b531752067414d4d6e6568f90')
        )

并创建一个复合索引(soha_id, info_hash)以使其快速工作。

MySQL还可以使用 将两个索引的结果合并在一起index_merge，因此即使您不创建复合索引，您也有可能在第二个查询的计划中看到这一点。