@dbsnake-用合适的函数索引来避免看似无法避免的全表扫描
昨天听了@dbsnake的SQL方法论,感觉比第一次要更有感觉,希望对实际工作能有帮助。
昨天讲到一处利用reverse函数建立索引,避免全表扫描的case,颇有感触,拿出来试一下。
SQL> create table rev (id number, name varchar2(5));
Table created.
SQL> select * from rev;
ID NAME
---------- -----
1 abc
2 bc
3 c
SQL> create index rev_idx0 on rev(name);
Index created.
SQL> set autot on
SQL> select id, name from rev where name like '%bc';
ID NAME
---------- -----
1 abc
2 bc
Execution Plan
----------------------
Plan hash value: 3205185662
--------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------
| 0 | SELECT STATEMENT | | 2 | 34 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| REV | 2 | 34 | 3 (0)| 00:00:01 |
--------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("NAME" LIKE '%bc')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------
5 recursive calls
0 db block gets
16 consistent gets
0 physical reads
0 redo size
633 bytes sent via SQL*Net to client
492 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
这里建立了name的B树索引,但由于使用了%bc为条件,所以不会用索引,这里用了全表扫描。
如何能让%bc条件使用索引呢?这里讲到%bc不能用索引的原因是因为索引键值按照索引二进制的顺序排序,%在前就无法精确定位,因此无法使用索引。既然%在后面可以使用索引,那就想办法将%的条件放在后面组织。
SQL> create index rev_idx on rev(reverse(name));
Index created.
SQL> select id, name from rev where reverse(name) like reverse('%bc');
ID NAME
---------- -----
2 bc
1 abc
Execution Plan
----------------------
Plan hash value: 2418054352
---------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 34 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| REV | 2 | 34 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | REV_IDX | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access(REVERSE("NAME") LIKE 'cb%')
filter(REVERSE("NAME") LIKE 'cb%')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------
28 recursive calls
0 db block gets
14 consistent gets
0 physical reads
0 redo size
633 bytes sent via SQL*Net to client
492 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
这里用了reverse函数,相当于方向匹配字符串bc,这样就将前面的%放到了后面。从执行计划看cost从3变为2。
注:
这里可以看到无论哪次执行,物理读都是0,原因我觉得就是第一次执行过一个select * from rev;,因为数据量比较小,第一次select之后,记录就从data file缓存到buffer cache,即使根据LRU算法,负载不是太大的DB,很快的时间内这些数据还可能在其中,没被age out,所以再次执行SQL时,就可能物理读是0。
总结:
以上的示例就是@dbsnake讲的“用合适的函数索引来避免看似无法避免的全表扫描“。