导读：有的时候，使用错误的索引会导致Oracle数据库的效率明显下降，通过一些方法或者是技巧可以有效的避免这个问题，下文中就为大家带来避免使用错误的数据库索引的，以提高Oracle数据库的工作效率。
这个例子中,如果我想使用idx_a而不是idx_b.
SQL> create table test 2 (a int,b int,c int,d int); Table created. SQL> begin 2 for i in 1..50000 3 loop 4 insert into mytest values(i,i,i,i); 5 end loop; 6 commit; 7 end; 8 / PL/SQL procedure successfully completed. SQL> create index idx_a on mytest(a,b,c); Index created. SQL> create index idx_b on mytest(b); Index created.
如表mytest,有字段a,b,c,d，在a,b,c上建立联合索引idx_a(a,b,c)，在b上单独建立了一个索引idx_b(b)。
在正常情况下，where a=? and b=? and c=?会用到索引idx_a，where b=?会用到索引idx_b
比如：
SQL> analyze table mytest compute statistics; Table analyzed. SQL> select num_Rows from user_tables where table_name=’MYTEST’; NUM_ROWS ———- 50000 SQL> select distinct_keys from user_indexes where index_name=’IDX_A’; DISTINCT_KEYS ————- 50000 SQL> set autotrace traceonly SQL> select d from mytest 2 where a=10 and b=10 and c=10; Execution Plan ———————————————————- Plan hash value: 1542625214 ——————————————————————————– —— | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ——————————————————————————– ——
| 0 | SELECT STATEMENT | | 1 | 16 | 2 (0)| 00:0 0:01 | | 1 | TABLE ACCESS BY INDEX ROWID| MYTEST | 1 | 16 | 2 (0)| 00:0 0:01 | |* 2 | INDEX RANGE SCAN | IDX_A | 1 | | 1 (0)| 00:0 0:01 | ——————————————————————————– —— Predicate Information (identified by operation id): ————————————————— 2 – access("A"=10 AND "B"=10 AND "C"=10) Statistics ———————————————————- 1 recursive calls 0 db block gets 4 consistent gets 0 physical reads 0 redo size 508 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) 1 rows processed SQL> select d from mytest 2 where b=500; Execution Plan ———————————————————- Plan hash value: 530004086 ——————————————————————————– —— | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ——————————————————————————– —— | 0 | SELECT STATEMENT | | 1 | 8 | 2 (0)| 00:0 0:01 | | 1 | TABLE ACCESS BY INDEX ROWID| MYTEST | 1 | 8 | 2 (0)| 00:0 0:01 | |* 2 | INDEX RANGE SCAN | IDX_B | 1 | | 1 (0)| 00:0 0:01 | ——————————————————————————– —— Predicate Information (identified by operation id): ————————————————— 2 – access("B"=500) Statistics ———————————————————- 1 recursive calls 0 db block gets 4 consistent gets 0 physical reads 0 redo size 508 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) 1 rows processed
#p#
但是在这样一个条件下：where a=? and b=? and c=? group by b会用到哪个索引呢?在索引的分析数据不正确(很长时间没有分析)或根本没有分析数据的情况下，oracle往往会使用索引idx_b。通过执行计划的分析，这个索引的使用，将大大耗费查询时间。
比如在索引有统计信息，分析数据正确的情况下：
SQL> select max(d) from mytest 2 where a=50 and b=50 and c=50 3 group by b; Execution Plan ———————————————————- Plan hash value: 422688974 ——————————————————————————– ——- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Tim e | ——————————————————————————– ——- | 0 | SELECT STATEMENT | | 1 | 16 | 2 (0)| 00: 00:01 | | 1 | SORT GROUP BY NOSORT | | 1 | 16 | 2 (0)| 00: 00:01 | | 2 | TABLE ACCESS BY INDEX ROWID| MYTEST | 1 | 16 | 2 (0)| 00: 00:01 | |* 3 | INDEX RANGE SCAN | IDX_A | 1 | | 1 (0)| 00: 00:01 | ——————————————————————————– ——- Predicate Information (identified by operation id): ————————————————— 3 – access("A"=50 AND "B"=50 AND "C"=50) Statistics ———————————————————- 1 recursive calls 0 db block gets 3 consistent gets 0 physical reads 0 redo size 513 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) 1 rows processed
但如果索引分析数据不正确:
SQL> select num_rows from user_tables 2 where table_name=’MYTEST’; NUM_ROWS ———- 50000 SQL> analyze index idx_a delete statistics; Index analyzed. SQL> analyze index idx_b delete statistics; Index analyzed. SQL> select distinct_keys from user_indexes 2 where index_name in (‘IDX_A’,’IDX_B’); DISTINCT_KEYS ————- SQL> select max(d) from mytest where a=50 and b=50 and c=50 group by b; Execution Plan ———————————————————- Plan hash value: 3925507835 ——————————————————————————– ——- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Tim e | ——————————————————————————– ——- | 0 | SELECT STATEMENT | | 1 | 16 | 2 (0)| 00: 00:01 | | 1 | SORT GROUP BY NOSORT | | 1 | 16 | 2 (0)| 00: 00:01 | |* 2 | TABLE ACCESS BY INDEX ROWID| MYTEST | 1 | 16 | 2 (0)| 00: 00:01 | |* 3 | INDEX RANGE SCAN | IDX_B | 1 | | 1 (0)| 00: 00:01 | ——————————————————————————– ——- Predicate Information (identified by operation id): ————————————————— 2 – filter("A"=50 AND "C"=50) 3 – access("B"=50) Statistics ———————————————————- 0 recursive calls 0 db block gets 3 consistent gets 0 physical reads 0 redo size 513 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) 1 rows processed
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我们可以通过如下的技巧避免使用idx_b，而使用idx_a。
where a=? and b=? and c=? group by b||” –如果b是字符类型
where a=? and b=? and c=? group by b+0 –如果b是数字类型
通过这样简单的改变，往往可以是查询时间提交很多倍
当然，我们也可以使用no_index提示，相信很多人没有用过，也是一个不错的方法：
SQL> select /*+ no_index(mytest,idx_b) */ max(d) from mytest where a=50 and b=50 and c=50 group by b; Execution Plan ———————————————————- Plan hash value: 422688974 ——————————————————————————– ——- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Tim e | ——————————————————————————– ——- | 0 | SELECT STATEMENT | | 1 | 16 | 9 (0)| 00: 00:01 | | 1 | SORT GROUP BY NOSORT | | 1 | 16 | 9 (0)| 00: 00:01 | | 2 | TABLE ACCESS BY INDEX ROWID| MYTEST | 1 | 16 | 9 (0)| 00: 00:01 | |* 3 | INDEX RANGE SCAN | IDX_A | 500 | | 1 (0)| 00: 00:01 | ——————————————————————————– ——- Predicate Information (identified by operation id): ————————————————— 3 – access("A"=50 AND "B"=50 AND "C"=50) Statistics ———————————————————- 1 recursive calls 0 db block gets 3 consistent gets 0 physical reads 0 redo size 513 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) 1 rows processed
上文中主要是以代码的形式为大家讲解的，看起来可能是不太容易理解，大家要深入其中去学习，这个技巧是非常实用的，希望大家能够从中收获。
