oracle like模糊查询不能走索引?
2019-06-07 21:31
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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
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