组合索引经要素!

/* 1.适用在单独查询返回记录很多，组合查询后忽然返回记录很少的情况:

   比如where 学历=硕士以上 返回不少的记录

   比如where 职业=收银员 同样返回不少的记录

   于是无论哪个条件查询做索引，都不合适。

   可是，如果学历为硕士以上，同时职业又是收银员的，返回的就少之又少了。

   于是联合索引就可以这么开始建了。

*/   

/* 2.组合查询的组合顺序，要考虑单独的前缀查询情况（否则单独前缀查询的索引不能生效或者只能用到跳跃索引）

   比如你在建id,object_type的联合索引时，要看考虑是单独where id=xxx查询的多，还是单独where object_type查询的多。

   这里细节就暂时略去了，在案例的部分中还有描述

*/

--3.仅等值无范围查询时，组合索引顺序不影响性能（比如where col1=xxx and col2=xxx，无论COL1+COL2组合还是COL2+COL1组合）

drop table t purge;

create table t as select * from dba_objects;

insert into t select * from t;

insert into t select * from t;

insert into t select * from t;

update t set object_id=rownum ;

commit;

create index idx_id_type on t(object_id,object_type);

create index idx_type_id on t(object_type,object_id);

set autotrace off

alter session set statistics_level=all ;

set linesize 366

select /*+index(t,idx_id_type)*/ * from  t  where object_id=20  and object_type='TABLE';

select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

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| Id  | Operation                   | Name        | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

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

|   0 | SELECT STATEMENT            |             |      1 |        |      1 |00:00:00.01 |       5 |

|   1 |  TABLE ACCESS BY INDEX ROWID| T           |      1 |     57 |      1 |00:00:00.01 |       5 |

|*  2 |   INDEX RANGE SCAN          | IDX_ID_TYPE |      1 |      9 |      1 |00:00:00.01 |       4 |

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

select /*+index(t,idx_type_id)*/ * from  t  where object_id=20  and object_type='TABLE';

select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

Plan hash value: 3420768628

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| Id  | Operation                   | Name        | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

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

|   0 | SELECT STATEMENT            |             |      1 |        |      1 |00:00:00.01 |       5 |

|   1 |  TABLE ACCESS BY INDEX ROWID| T           |      1 |     57 |      1 |00:00:00.01 |       5 |

|*  2 |   INDEX RANGE SCAN          | IDX_TYPE_ID |      1 |      9 |      1 |00:00:00.01 |       4 |

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--4.组合索引最佳顺序一般是将列等值查询的列置前。（测试组合索引在条件是不等的情况下的情况，条件经常是不等的，要放在后面，让等值的在前面）

select /*+index(t,idx_id_type)*/ *  from   t where object_id>=20 and object_id<2000 and object_type='TABLE';

select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

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

| Id  | Operation                   | Name        | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

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

|   0 | SELECT STATEMENT            |             |      1 |        |    469 |00:00:00.01 |      86 |

|   1 |  TABLE ACCESS BY INDEX ROWID| T           |      1 |     14 |    469 |00:00:00.01 |      86 |

|*  2 |   INDEX RANGE SCAN          | IDX_ID_TYPE |      1 |      1 |    469 |00:00:00.01 |      40 |

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

select /*+index(t,idx_type_id)*/ *  from  t  where object_id>=20 and object_id<2000   and object_type='TABLE';

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

| Id  | Operation                   | Name        | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

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

|   0 | SELECT STATEMENT            |             |      1 |        |    469 |00:00:00.01 |      81 |

|   1 |  TABLE ACCESS BY INDEX ROWID| T           |      1 |    469 |    469 |00:00:00.01 |      81 |

|*  2 |   INDEX RANGE SCAN          | IDX_TYPE_ID |      1 |    469 |    469 |00:00:00.01 |      35 |

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--5.注意组合索引与组合条件中关于IN 的优化(将会在案例部分描述，展现结果就不在这里贴出了）

--案例1

UPDATE t SET OBJECT_ID=20 WHERE ROWNUM<=26000;

UPDATE t SET OBJECT_ID=21 WHERE OBJECT_ID<>20;

COMMIT;

set linesize 1000

set pagesize 1

alter session set statistics_level=all ;

select  /*+index(t,idx1_object_id)*/ * from t  where object_TYPE='TABLE'  AND OBJECT_ID >= 20 AND OBJECT_ID<= 21;

--6.案例2

--依然是关于IN的优化 (col1,col2,col3的索引情况，如果没有为COL2赋予查询条件时，COL3只能起到检验作用)

drop table t purge;

create table t as select * from dba_objects;

UPDATE t SET OBJECT_ID=20 WHERE ROWNUM<=26000;

UPDATE t SET OBJECT_ID=21 WHERE OBJECT_ID<>20;

Update t set object_id=22 where rownum<=10000;

COMMIT;

create index idx_union on t(object_type,object_id,owner);

set autotrace off

alter session set statistics_level=all ;

set linesize 1000

select * from t where object_type='VIEW' and OWNER='LJB';

select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

--这里就略去了展现结果，在案例中有描述。

select /*+INDEX(T,idx_union)*/ * from t T where object_type='VIEW' and OBJECT_ID IN (20,21,22) AND OWNER='LJB';

select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));