大家好,又见面了,我是全栈君。
假设一张表上有十几个索引,你有什么感受?显然会拖慢增、删、改的速度。不要指望开发者能建好索引。我的处理方法是先监控非常长的一段时间。看哪些索引没实用到,然后删除。
但删除以后,假设发现某一天实用,那又要又一次建,假设是大表。那就有些麻烦。如今11g提供一个新特性,不可见索引。能够建索引设置为不可见索引。CBO在评估运行计划的时候会忽略它,假设须要的时候。设置回来就可以。
另一种用途,你在调试一条SQL语句,要建一个索引測试。而你不想影响其它的会话,用不可见索引正是时候。
SQL> drop table test purge;
SQL> create table test as select * from dba_objects;
SQL> create index ind_t_object_id on test(object_id);
SQL> exec dbms_stats.gather_table_stats(user,’test’,cascade => true);
SQL> set autotrace traceonly
SQL> select * from test where object_id = 10;
运行计划
———————————————————-
Plan hash value: 255872589
———————————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
———————————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 100 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 100 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_T_OBJECT_ID | 1 | | 1 (0)| 00:00:01 |
———————————————————————————————–
Predicate Information (identified by operation id):
—————————————————
2 – access(“OBJECT_ID”=10)
统计信息
———————————————————-
1 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
1195 bytes sent via SQL*Net to client
337 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> alter index ind_t_object_id invisible;
SQL> select * from test where object_id = 10;
运行计划
———————————————————-
Plan hash value: 1357081020
————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 100 | 209 (1)| 00:00:03 |
|* 1 | TABLE ACCESS FULL| TEST | 1 | 100 | 209 (1)| 00:00:03 |
————————————————————————–
Predicate Information (identified by operation id):
—————————————————
1 – filter(“OBJECT_ID”=10)
统计信息
———————————————————-
196 recursive calls
0 db block gets
567 consistent gets
0 physical reads
0 redo size
1195 bytes sent via SQL*Net to client
337 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
6 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select /*+ index(test ind_t_object_id)*/ * from test where object_id = 10;
运行计划
———————————————————-
Plan hash value: 1357081020
————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 100 | 209 (1)| 00:00:03 |
|* 1 | TABLE ACCESS FULL| TEST | 1 | 100 | 209 (1)| 00:00:03 |
————————————————————————–
Predicate Information (identified by operation id):
—————————————————
1 – filter(“OBJECT_ID”=10)
统计信息
———————————————————-
1 recursive calls
0 db block gets
544 consistent gets
0 physical reads
0 redo size
1195 bytes sent via SQL*Net to client
337 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
–让数据库看到不可见索引,能够通过改变一个參数
SQL> alter session set optimizer_use_invisible_indexes = true;
SQL> select * from test where object_id = 10;
运行计划
———————————————————-
Plan hash value: 255872589
———————————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
———————————————————————————————–
| 0 | SELECT STATEMENT | | 1 | 100 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 100 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_T_OBJECT_ID | 1 | | 1 (0)| 00:00:01 |
———————————————————————————————–
Predicate Information (identified by operation id):
—————————————————
2 – access(“OBJECT_ID”=10)
统计信息
———————————————————-
1 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
1195 bytes sent via SQL*Net to client
337 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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