oracle like模糊查询不能走索引？

Posted 2021-12-28 mzq123

这里要纠正一个网上很多教程说的模糊匹配不能走索引的说法，因为在看《收获，不止SQL优化》一书，里面举例说到了，并且自己也跟着例子实践了一下，确实like一些特殊情况也是可以走索引的

例子来自《收获，不止SQL优化》一书，实践准备：

//建表，注意要非空数据
drop table t purge;
create table t as select * from dba_objects where object_id is not null;

select * from t;

//更新数据并建索引，用来测试
update t set object_id=rownum;

update t set object_name=‘AAALJB‘ where object_id=8;

update t set object_name=‘LJBAAA‘ where object_id=10;

create index idx_object_name on t(object_name);

用set autotrace on用来打印执行计划，这里注意了，用LJB%去模糊匹配，然后观察执行计划，发现是索引范围扫描INDEX RANGE SCAN 的，因为去匹配LJB开头的数据，索引是可以范围查询并匹配到，所以是能走范围索引扫描的，所以网上的说法是不全面的

SQL> set autotrace on
SQL> select object_id,object_name from t where object_name like ‘LJB%‘;

 OBJECT_ID
----------
OBJECT_NAME
--------------------------------------------------------------------------------

        10
LJBAAA



执行计划
----------------------------------------------------------
Plan hash value: 1138138579

--------------------------------------------------------------------------------

---------------

| Id  | Operation                   | Name            | Rows  | Bytes | Cost (%C

PU)| Time     |

--------------------------------------------------------------------------------

---------------

|   0 | SELECT STATEMENT            |                 |     1 |    79 |     4
(0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| T               |     1 |    79 |     4
(0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN          | IDX_OBJECT_NAME |     1 |       |     3
(0)| 00:00:01 |

--------------------------------------------------------------------------------

---------------


Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("OBJECT_NAME" LIKE ‘LJB%‘)
       filter("OBJECT_NAME" LIKE ‘LJB%‘)

Note
-----
   - dynamic sampling used for this statement (level=2)

SQL>

上面列举了，能走索引的例子，然后改一下用%LJB去匹配，看看能不能走索引？

SQL> set autotrace on
SQL> select object_id,object_name from t where object_name like ‘%LJB‘;

 OBJECT_ID
----------
OBJECT_NAME
--------------------------------------------------------------------------------

         8
AAALJB



执行计划
----------------------------------------------------------
Plan hash value: 1601196873

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    12 |   948 |   288   (1)| 00:00:04 |
|*  1 |  TABLE ACCESS FULL| T    |    12 |   948 |   288   (1)| 00:00:04 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("OBJECT_NAME" IS NOT NULL AND "OBJECT_NAME" LIKE ‘%LJB‘)

Note
-----
   - dynamic sampling used for this statement (level=2)


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       1032  consistent gets
          0  physical reads
          0  redo size
        503  bytes sent via SQL*Net to client
        419  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

SQL>

例子可以看出是全表扫描的，不走索引，因为%LJB这种匹配，索引不能确认唯一性，同样的%LJB%去匹配也是不走索引的