oracle细节之like模糊查询不能走索引？

本是古典何须时尚 2021-09-19 22:40 388阅读 0赞

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