Oracle 11g新特性invisible index(不可见的索引)[通俗易懂]

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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