利用虚拟索引（Virtual Index）优化数据库的案例分析

当我们在对生产库做优化的时候，主要就是对SQL语句的优化，包括语句的等价改写等，但其中很大一部分情况，又与索引有关。如果能合理利用合适的索引，可以使原本走全表扫描产生的逻辑读大大降低，提高数据库的性能。由于Oracle数据库中的索引本身就要占用磁盘空间，维护索引需要一定的开销，如何才能知道创建某个索引，会给数据带来性能的提升，而又不至于判断失误，创建了一个不恰当的索引，最后又不得不删除呢？这种情况下，我们可以利用Oralce提供的虚拟索引，即nosegment索引，它并不占用磁盘资源，只是在数据字典中增加一个定义。它为DBA在创建索引对提升数据库性能的方面提供了一定的参考。下面来看具体测试和分析：

SQL> startup

ORACLE instance started.

Total System Global Area  835104768 bytes

Fixed Size                  2232960 bytes

Variable Size             675286400 bytes

Database Buffers          155189248 bytes

Redo Buffers                2396160 bytes

Database mounted.

Database opened.

--本测试在11.2.0.3.0环境，与10g略有不同

SQL> select * from v$version;

BANNER

--------------------------------------------------------------------------------

Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production

PL/SQL Release 11.2.0.3.0 - Production

CORE    11.2.0.3.0      Production

TNS for Linux: Version 11.2.0.3.0 - Production

NLSRTL Version 11.2.0.3.0 - Production

--创建测试表fakeind并插入数据

SQL> drop table fakeind_test;

drop table fakeind_test

           *

ERROR at line 1:

ORA-00942: table or view does not exist

SQL> create table fakeind_test as select * from dba_objects;

Table created.

SQL> insert into fakeind_test select * from fakeind_test;

75540 rows created.

SQL> /

151080 rows created.

SQL> /

302160 rows created.

SQL> select count(*) from fakeind_test;

  COUNT(*)

----------

    604320

--开始测试，执行查询

SQL> set line 130 pages 130

SQL> select object_id,object_name from fakeind_test where object_id in(select distinct object_id from fakeind_test where object_id>44500 and object_id<45000);

3992 rows selected.

Execution Plan

----------------------------------------------------------

Plan hash value: 1190425891

-------------------------------------------------------------------------------------

| Id  | Operation            | Name         | Rows  | Bytes | Cost (%CPU)| Time     |

-------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT     |              |  3816 |   160K|  4667   (1)| 00:00:57 |

|*  1 |  HASH JOIN RIGHT SEMI|              |  3816 |   160K|  4667   (1)| 00:00:57 |

|   2 |   VIEW               | VW_NSO_1     |  3819 | 49647 |  2333   (1)| 00:00:28 |

|*  3 |    TABLE ACCESS FULL | FAKEIND_TEST |  3819 | 19095 |  2333   (1)| 00:00:28 |

|   4 |   TABLE ACCESS FULL  | FAKEIND_TEST |   604K|    17M|  2331   (1)| 00:00:28 |

-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   1 - access("OBJECT_ID"="OBJECT_ID")

   3 - filter("OBJECT_ID">44500 AND "OBJECT_ID"<45000)

Statistics

----------------------------------------------------------

         23  recursive calls

          0  db block gets

      17436  consistent gets

          0  physical reads

          0  redo size

     144488  bytes sent via SQL*Net to client

       3445  bytes received via SQL*Net from client

        268  SQL*Net roundtrips to/from client

          3  sorts (memory)

          0  sorts (disk)

       3992  rows processed

可以看到，用CTAS创建的测试表fakeind上目前并没有索引，因此在生成的执行计划中，该条SQL语句只能走全表扫描

--创建虚拟索引(在普通创建索引命令后加一个nosegmnet即可)

SQL> create index ind_fake_id on fakeind_test(object_id) nosegment;

Index created.

--设置隐含参数使虚拟索引生效

SQL> alter session set "_use_nosegment_indexes"=true;  --注意必须要写双引号，单引号不行

Session altered.

SQL> set autot off

--查看表是否被分析过

SQL> select table_name,last_analyzed from dba_tables where table_name='FAKEIND_TEST';

TABLE_NAME                     LAST_ANALYZED

------------------------------ ------------------

FAKEIND_TEST

--收集测试表的统计信息

SQL> exec dbms_stats.gather_table_stats(ownname=>'SYS',tabname=>'FAKEIND_TEST',degree=>4,estimate_percent=>100,cascade=>true);

PL/SQL procedure successfully completed.

--再次确认表的分析情况

SQL> select table_name,last_analyzed from dba_tables where table_name='FAKEIND_TEST';

TABLE_NAME                     LAST_ANALYZED

------------------------------ ------------------

FAKEIND_TEST                   17-SEP-14

--再次查询测试表

SQL> set autot trace

SQL> select object_id,object_name from fakeind_test where object_id in(select distinct object_id from fakeind_test where object_id>44500 and object_id<45000);

3992 rows selected.

Execution Plan

----------------------------------------------------------

Plan hash value: 2531911586

-----------------------------------------------------------------------------------------------

| Id  | Operation                      | Name         | Rows  | Bytes | Cost (%CPU)| Time     |

-----------------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT               |              |  3904 |   308K|    12  (17)| 00:00:01 |

|   1 |  VIEW                          | VM_NWVW_2    |  3904 |   308K|    12  (17)| 00:00:01 |

|   2 |   HASH UNIQUE                  |              |  3904 |   179K|    12  (17)| 00:00:01 |

|*  3 |    HASH JOIN                   |              |  3904 |   179K|    11  (10)| 00:00:01 |

|*  4 |     INDEX RANGE SCAN           | IND_FAKE_ID  |  3819 | 19095 |     2   (0)| 00:00:01 |

|   5 |     TABLE ACCESS BY INDEX ROWID| FAKEIND_TEST |  3819 |   156K|     8   (0)| 00:00:01 |

|*  6 |      INDEX RANGE SCAN          | IND_FAKE_ID  |  3819 |       |     2   (0)| 00:00:01 |

-----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   3 - access("OBJECT_ID"="OBJECT_ID")

   4 - access("OBJECT_ID">44500 AND "OBJECT_ID"<45000)

   6 - access("OBJECT_ID">44500 AND "OBJECT_ID"<45000)

Statistics

----------------------------------------------------------

          1  recursive calls

          0  db block gets

      17418  consistent gets

          0  physical reads

          0  redo size

     144488  bytes sent via SQL*Net to client

       3445  bytes received via SQL*Net from client

        268  SQL*Net roundtrips to/from client

          1  sorts (memory)

          0  sorts (disk)

       3992  rows processed

SQL> set autot off

此时利用虚拟索引获得的执行计划中，COST从之前的4000多降低到12，执行时间也从57s到1s，由此可以判断，当加上真实索引后，性能会大大提高。

--创建真实索引

SQL> create index ind_real_id on fakeind_test(object_id);

Index created.

SQL> set autot trace

SQL> select object_id,object_name from fakeind_test where object_id in(select distinct object_id from fakeind_test where object_id>45500 and object_id<50000);

35992 rows selected.

Execution Plan

----------------------------------------------------------

Plan hash value: 2531911586

-------------------------------------------------------------------------------------------------------

| Id  | Operation                      | Name         | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |

-------------------------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT               |              | 41816 |  3307K|       |   548   (1)| 00:00:07 |

|   1 |  VIEW                          | VM_NWVW_2    | 41816 |  3307K|       |   548   (1)| 00:00:07 |

|   2 |   HASH UNIQUE                  |              | 41816 |  1919K|  2472K|   548   (1)| 00:00:07 |

|*  3 |    HASH JOIN                   |              | 41816 |  1919K|       |    53   (2)| 00:00:01 |

|*  4 |     INDEX RANGE SCAN           | IND_FAKE_ID  | 34375 |   167K|       |     3   (0)| 00:00:01 |

|   5 |     TABLE ACCESS BY INDEX ROWID| FAKEIND_TEST | 34375 |  1409K|       |    49   (0)| 00:00:01 |

|*  6 |      INDEX RANGE SCAN          | IND_FAKE_ID  | 34375 |       |       |     3   (0)| 00:00:01 |

-------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   3 - access("OBJECT_ID"="OBJECT_ID")

   4 - access("OBJECT_ID">45500 AND "OBJECT_ID"<50000)

   6 - access("OBJECT_ID">45500 AND "OBJECT_ID"<50000)

Statistics

----------------------------------------------------------

          1  recursive calls

          0  db block gets

      11017  consistent gets

         82  physical reads

          0  redo size

    1293055  bytes sent via SQL*Net to client

      26908  bytes received via SQL*Net from client

       2401  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

      35992  rows processed

虽然创建了真实索引，但数据库却仍旧在用虚拟索引，此时COST和TIME反而还上去了一点，那么需要先禁用虚拟索引

SQL> alter session set "_use_segment_indexes"=false;

--禁用虚拟索引后继续查看刚才的SQL

SQL> select object_id,object_name from fakeind_test where object_id in(select distinct object_id from fakeind_test where object_id>45500 and object_id<50000);

35992 rows selected.

Execution Plan

----------------------------------------------------------

Plan hash value: 750753197

-------------------------------------------------------------------------------------

| Id  | Operation            | Name         | Rows  | Bytes | Cost (%CPU)| Time     |

-------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT     |              | 34375 |  1443K|  2414   (1)| 00:00:29 |

|*  1 |  HASH JOIN RIGHT SEMI|              | 34375 |  1443K|  2414   (1)| 00:00:29 |

|   2 |   VIEW               | VW_NSO_1     | 34375 |   436K|    79   (0)| 00:00:01 |

|*  3 |    INDEX RANGE SCAN  | IND_REAL_ID  | 34375 |   167K|    79   (0)| 00:00:01 |

|   4 |   TABLE ACCESS FULL  | FAKEIND_TEST |   604K|    17M|  2331   (1)| 00:00:28 |

-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   1 - access("OBJECT_ID"="OBJECT_ID")

   3 - access("OBJECT_ID">45500 AND "OBJECT_ID"<50000)

Statistics

----------------------------------------------------------

          0  recursive calls

          0  db block gets

      11017  consistent gets

          0  physical reads

          0  redo size

    1293055  bytes sent via SQL*Net to client

      26908  bytes received via SQL*Net from client

       2401  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

      35992  rows processed

虽然使用真实索引之后，性能提升并不如使用虚拟索引时那样多，但至少比最初没有索引的情况下，要快了将近28秒，COST也减少了将近一半，当真实索引建立完毕以后，可以对虚拟索引进行删除，以免白白占用一个对象名，删除语法和删除普通索引一致。

虚拟索引有几个要注意的地方：

--虚拟索引并不存在于dba_indexes视图

SQL> select index_name from dba_indexes where index_name='IND_FAKE_ID';

no rows selected

--无法创建与虚拟索引同名的真实索引

SQL> create index ind_fake_id on fakeind_test(object_name);

create index ind_fake_id on fakeind_test(object_name)

             *

ERROR at line 1:

ORA-00955: name is already used by an existing object

--无法使用alter命令来修改或重建索引

SQL> alter index ind_fake_id rename to ind_fake_name;

alter index ind_fake_id rename to ind_fake_name

*

ERROR at line 1:

ORA-08114: can not alter a fake index

SQL> alter index ind_fake_id rebuild;

alter index ind_fake_id rebuild

*

ERROR at line 1:

ORA-08114: can not alter a fake index

--查看虚拟索引的方法

SQL> set autot off

SQL> SELECT index_owner, index_name  

  2  FROM dba_ind_columns

  3  WHERE index_name NOT LIKE 'BIN$%' 

  4  MINUS

  5  SELECT owner, index_name 

  6  FROM dba_indexes;

INDEX_OWNER                    INDEX_NAME

------------------------------ ------------------------------

SYS                            IND_FAKE_ID

--drop table并不会直接删除虚拟索引，必须purge recyclebin或者显示drop index

SQL> conn zlm/oracle  --由于SYS用户默认没有recyclebin，这里用zlm用户做测试

Connected.

SQL> create table t1 as select * from dba_objects;

Table created.

SQL> create index ind_t1_id on t1(object_id) nosegment;

Index created.

SQL> drop table t1;

Table dropped.

SQL> SELECT index_owner, index_name  

  2  FROM dba_ind_columns

  3  WHERE index_name NOT LIKE 'BIN$%' 

  4  MINUS

  5  SELECT owner, index_name 

  6  FROM dba_indexes;

INDEX_OWNER                    INDEX_NAME

------------------------------ ------------------------------

ZLM                            IND_T1_ID    --尽管已经删除了T1表，但是虚拟索引仍然还在

SQL> purge recyclebin;

Recyclebin purged.

SQL> SELECT index_owner, index_name  

  2  FROM dba_ind_columns

  3  WHERE index_name NOT LIKE 'BIN$%' 

  4  MINUS

  5  SELECT owner, index_name 

  6  FROM dba_indexes;

no rows selected

--清空回收站后，虚拟索引才被彻底删除了，或者直接显示删除虚拟索引

SQL> drop index ind_t1_id;

总结：

这个测试其实并没有做的很完善，SQL语句选取得不好，正常情况下，原有语句所涉及的表至少且肯定会有一个主键索引，没有索引的表在OLTP生产库中是不太现实的。本文主要是起到一个抛砖引玉的作用，当我们面对一个数据库优化的场景，需要测试创建某个特定条件的索引是否会给系统带来性能提升，就可以借助虚拟索引来测试，因为在生产库中通常不允许随意创建索引，而维护索引对数据库而言也是一笔不小的开销，况且如果索引创建不当，可能使数据库性能更糟糕。