标签：00 可用 -- 索引 tlzl mysql where 列为
组合索引的第二列为in时是否可用走索引？当时面试直接把我问蒙了。一般来说in会等价改写成or，然后走索引。比如
where a in (1,2)
等价于
where a=1 or a=2在oracle中肯定会这样去改写，所以该走索引的还是会走索引。但是这个条件放在组合索引的第二列呢？
首先等价改写同样成立，比如
where a in (1,2) and b=0
等价于
where (a=1 or a=2) and b=0优化器是否会这样做呢？这只有测试了才知道
oracle测试
--单列索引
create index idx_tlzl on tlzl(a);
1* select * from tlzl where a in (1,2)
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 40 | 1 (0)| 00:00:01 |
| 1 | INLIST ITERATOR | | | | | |
| 2 | TABLE ACCESS BY INDEX ROWID| TLZL | 2 | 40 | 1 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | IDX_TLZL | 4 | | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("A"=1 OR "A"=2)
--复合索引
create index idx_tlzl2 on tlzl(b,a);
--仅指定第二列的条件时，oracle选择走全表扫描
--其实oracle还有索引跳跃扫描的方案，因为b列只有1个值，全表扫描的效率还是要高一些
SQL> select * from tlzl where a in (1,2);
Execution Plan
----------------------------------------------------------
Plan hash value: 925964718
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 40 | 2 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| TLZL | 2 | 40 | 2 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("A"=1 OR "A"=2)
--oracle做了等价改写，in改写成or
--第一列和第二列都有条件时，可以走索引，索引范围扫描
--且做了等价改写
SQL> select * from tlzl where a in (1,2) and b='a';
Execution Plan
----------------------------------------------------------
Plan hash value: 3195115994
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 40 | 2 (0)| 00:00:01 |
| 1 | INLIST ITERATOR | | | | | |
|* 2 | INDEX RANGE SCAN| IDX_TLZL2 | 2 | 40 | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("B"='a' AND ("A"=1 OR "A"=2))
mysql测试
--单列索引
mysql> create index idx_tlzl on tlzl(a);
--单列索引时，条件为in时，mysql可以走索引，走索引范围扫描range
mysql> explain select * from tlzl where a in (1,2);
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | tlzl | NULL | range | idx_tlzl | idx_tlzl | 5 | NULL | 2 | 100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.06 sec)
--符合索引
mysql> create index idx_tlzl2 on tlzl(b,a);
--符合索引只有第二列的条件
--mysql走了索引，但是是index，表示索引全扫描
mysql> explain select * from tlzl where a in (1,2);
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| 1 | SIMPLE | tlzl | NULL | index | NULL | idx_tlzl2 | 38 | NULL | 1000 | 20.00 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.03 sec)
--符合索引有第一列和第二列的条件
--mysql可以走索引，索引范围扫描
mysql> explain select * from tlzl where a in (1,2) and b='a';
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
| 1 | SIMPLE | tlzl | NULL | range | idx_tlzl2 | idx_tlzl2 | 38 | NULL | 2 | 100.00 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+-----------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.03 sec)
总结：
1.无论是理论上还是实际上，组合索引的第二列为in是可以走索引的
2.有些细枝末节确实不太容易提前测试到，但是要相信自己的逻辑推理判断
标签：00,可用,--,索引,tlzl,mysql,where,列为 来源： https://blog.csdn.net/qq_40687433/article/details/116640754
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