【DB笔试面试572】在Oracle中,模糊查询可以使用索引吗?
- 2019 年 10 月 10 日
- 筆記
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题目部分
在Oracle中,模糊查询可以使用索引吗?
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答案部分
分为以下几种情况:
(1)若SELECT子句只检索索引字段,那么模糊查询可以使用索引,例如,“SELECT ID FROM TB WHERE ID LIKE '%123%';”可以使用索引。
(2)若SELECT子句不只检索索引字段还检索其它非索引字段,那么分为以下几种情况:
① 模糊查询形如“WHERE COL_NAME LIKE 'ABC%';”可以用到索引。
② 模糊查询形如“WHERE COL_NAME LIKE '%ABC';”不能使用索引,但是可以通过REVERSE函数来创建函数索引才能使用到索引。
③ 模糊查询形如“WHERE COL_NAME LIKE '%ABC%';”不能使用索引,但是,如果所查询的字符串有一定的规律的的话,那么还是可以使用到索引的,分以下几种情况:
a. 如果字符串ABC始终从原字符串的某个固定位置出现,那么可以创建SUBSTR函数索引进行优化。
b. 如果字符串ABC始终从原字符串结尾的某个固定位置出现,那么可以创建函数组合索引进行优化。
c. 如果字符串ABC在原字符串中位置不固定,那么可以通过改写SQL进行优化。改写的方法主要是通过先使用子查询查询出需要的字段,然后在外层嵌套,这样就可以使用到索引了。
④ 建全文索引后使用CONTAINS也可以用到域索引。
示例代码如下所示:
LHR@orclasm > CREATE TABLE TB_LHR_20160518_02 AS SELECT * FROM DBA_TABLES; Table created. LHR@orclasm > CREATE INDEX IDX_TB_LHR_20160518_02 ON TB_LHR_20160518_02(TABLE_NAME); Index created. LHR@orclasm > SET AUTOT ON LHR@orclasm > SELECT OWNER,TABLE_NAME FROM TB_LHR_20160518_02 WHERE TABLE_NAME LIKE 'AA%'; ------------------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 34 | 1 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| TB_LHR_20160518_02 | 1 | 34 | 1 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX_TB_LHR_20160518_02 | 1 | | 1 (0)| 00:00:01 | ------------------------------------------------------------------------------------------------------ LHR@orclasm > SELECT OWNER,TABLE_NAME FROM TB_LHR_20160518_02 WHERE TABLE_NAME LIKE '%AA%'; ---------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 34 | 31 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| TB_LHR_20160518_02 | 1 | 34 | 31 (0)| 00:00:01 | ---------------------------------------------------------------------------------------- --如果只查询索引字段,那么必然走索引: LHR@orclasm > SELECT TABLE_NAME FROM TB_LHR_20160518_02 WHERE TABLE_NAME LIKE '%AA%'; ----------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 17 | 7 (0)| 00:00:01 | |* 1 | INDEX FAST FULL SCAN| IDX_TB_LHR_20160518_02 | 1 | 17 | 7 (0)| 00:00:01 | ----------------------------------------------------------------------------------------------- --创建REVERSE函数索引: LHR@orclasm > CREATE INDEX IDX_TB_LHR_20160518_03 ON TB_LHR_20160518_02(REVERSE(TABLE_NAME)); Index created. LHR@orclasm > SELECT OWNER,TABLE_NAME FROM TB_LHR_20160518_02 WHERE REVERSE(TABLE_NAME) LIKE REVERSE('%AA'); ------------------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 152 | 7752 | 27 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| TB_LHR_20160518_02 | 152 | 7752 | 27 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX_TB_LHR_20160518_03 | 27 | | 2 (0)| 00:00:01 | ------------------------------------------------------------------------------------------------------ --如果被检索的字符串总是从第3个位置开始,那么可以先创建SUBSTR函数索引,再使用LIKE 'ABC%',如下所示: LHR@orclasm > CREATE INDEX IDX_TB_LHR_20160518_04 ON TB_LHR_20160518_02(SUBSTR(TABLE_NAME,3)); Index created. LHR@orclasm > SELECT OWNER,TABLE_NAME FROM TB_LHR_20160518_02 WHERE SUBSTR(TABLE_NAME,3) LIKE 'AA%'; ------------------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 152 | 9728 | 18 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| TB_LHR_20160518_02 | 152 | 9728 | 18 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX_TB_LHR_20160518_04 | 27 | | 2 (0)| 00:00:01 | ------------------------------------------------------------------------------------------------------ --如果字符串ABC始终从原字符串结尾的某个固定位置出现,那么可以创建函数组合索引进行优化。假如ABC从字符串倒数第五位出现,如下所示: LHR@orclasm > CREATE INDEX IDX_TB_LHR_20160518_05 ON TB_LHR_20160518_02(REVERSE(SUBSTR(TABLE_NAME,1,LENGTH(TABLE_NAME)-4))); Index created. LHR@orclasm > SELECT OWNER,TABLE_NAME FROM TB_LHR_20160518_02 WHERE REVERSE(SUBSTR(TABLE_NAME,1,LENGTH(TABLE_NAME)-4)) LIKE REVERSE('%AA'); ------------------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 159 | 9381 | 27 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| TB_LHR_20160518_02 | 159 | 9381 | 27 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IDX_TB_LHR_20160518_05 | 29 | | 2 (0)| 00:00:01 | ------------------------------------------------------------------------------------------------------ Predicate Information (identified by operation id): --------------------------------------------------- 2 - access(REVERSE(SUBSTR("TABLE_NAME",1,LENGTH("TABLE_NAME")-4)) LIKE 'AA%') filter(REVERSE(SUBSTR("TABLE_NAME",1,LENGTH("TABLE_NAME")-4)) LIKE 'AA%') --如果字符串ABC在原字符串中位置不固定,那么可以通过改写SQL进行优化。这种情况需要在LIKE的字段上存在普通索引的情况下,先使用子查询查询出需要的字段,然后在外层嵌套,这样就可以使用到索引了。如下所示: LHR@orclasm > SELECT OWNER,TABLE_NAME FROM TB_LHR_20160518_02 WHERE TABLE_NAME IN (SELECT TABLE_NAME FROM TB_LHR_20160518_02 WHERE TABLE_NAME LIKE '%AA%'); ------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 51 | 10 (10)| 00:00:01 | | 1 | NESTED LOOPS | | | | | | | 2 | NESTED LOOPS | | 1 | 51 | 10 (10)| 00:00:01 | | 3 | SORT UNIQUE | | 1 | 17 | 7 (0)| 00:00:01 | |* 4 | INDEX FAST FULL SCAN | IDX_TB_LHR_20160518_02 | 1 | 17 | 7 (0)| 00:00:01 | |* 5 | INDEX RANGE SCAN | IDX_TB_LHR_20160518_02 | 1 | | 1 (0)| 00:00:01 | | 6 | TABLE ACCESS BY INDEX ROWID| TB_LHR_20160518_02 | 1 | 34 | 2 (0)| 00:00:01 | ------------------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 4 - filter("TABLE_NAME" LIKE '%AA%') 5 - access("TABLE_NAME"="TABLE_NAME") filter("TABLE_NAME" LIKE '%AA%') Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 131 recursive calls 2 db block gets 221 consistent gets 111 physical reads 0 redo size 671 bytes sent via SQL*Net to client 519 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 7 sorts (memory) 0 sorts (disk) 3 rows processed
& 说明:
有关模糊查询LIKE的更多内容可以参考我的BLOG:http://blog.itpub.net/26736162/viewspace-2139039/
本文选自《Oracle程序员面试笔试宝典》,作者:李华荣。