【Oracle】含常数的复合索引

原来对于索引的认识只知道索引可以基于一个或者多个列，B-Tree索引不包含null，但有些情况下我们又需要通过where 列名 is null来查找一些数据，这时候数据库由于没办法使用索引就会使用全表扫描，导致执行效率低下，这时候我们可以通过使用含常数的复合索引来解决这个问题。
下面开始进行实验：
首先建立测试表

SYS@ORCL>create table test_objects nologging as select rownum id,a.* from dba_objects a where 1=2;

Table created.
插入500万条数据：

SYS@ORCL>declare

l_cnt number;

l_rows number:=&1;

begin

insert /*+ append */ into test_objects select rownum,a.* from dba_objects a;

l_cnt:=sql%rowcount;

commit;

while(l_cnt<l_rows)

loop

insert /*+ append */ into test_objects select rownum+l_cnt,

owner,object_name,subobject_name,

object_id,data_object_id,

object_type,created,last_ddl_time,

timestamp,status,temporary,

generated,secondary

from test_objects

where rownum<=l_rows-l_cnt;

l_cnt:=l_cnt+sql%rowcount;

commit;

end loop;

end; 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

22 /

Enter value for 1: 5000000

old 3: l_rows number:=&1;

new 3: l_rows number:=5000000;

PL/SQL procedure successfully completed.
在object_id列上建立一般的B-Tree索引：

SYS@ORCL>create index idx_oid_test_objects on test_objects(object_id);

Index created.
收集表信息：

SYS@ORCL>exec dbms_stats.gather_table_stats('SYS','TEST_OBJECTS');

PL/SQL procedure successfully completed.
测试一下索引能否正常使用：

SYS@ORCL>select count(*) from test_objects where object_id=52457;

COUNT(*)

----------

99

Execution Plan

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

Plan hash value: 3877533889

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

----------

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

Time |

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

----------

| 0 | SELECT STATEMENT | | 1 | 5 | 3 (0)|

00:00:01 |

| 1 | SORT AGGREGATE | | 1 | 5 | |

|

|* 2 | INDEX RANGE SCAN| IDX_OID_TEST_OBJECTS | 99 | 495 | 3 (0)|

00:00:01 |

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

----------

Predicate Information (identified by operation id):

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

2 - access("OBJECT_ID"=52457)

Statistics

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

1 recursive calls

0 db block gets

3 consistent gets

0 physical reads

0 redo size

411 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed
可以看到索引可以正常使用，下面我们来进入正题，当where条件中为object_id is null的时候会如何：

SYS@ORCL>select count(*) from test_objects where object_id is null;

COUNT(*)

----------

0

Execution Plan

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

Plan hash value: 3799704240

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

---

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

|

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

---

| 0 | SELECT STATEMENT | | 1 | 5 | 16612 (2)| 00:03:2

0 |

| 1 | SORT AGGREGATE | | 1 | 5 | |

|

|* 2 | TABLE ACCESS FULL| TEST_OBJECTS | 1 | 5 | 16612 (2)| 00:03:2

0 |

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

---

Predicate Information (identified by operation id):

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

2 - filter("OBJECT_ID" IS NULL)

Statistics

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

1 recursive calls

0 db block gets

74808 consistent gets

74730 physical reads

904 redo size

410 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed
因为B-Tree索引不会记录null所以只能使用全表扫描。产生了大量的consistent gets下面我们来建立含常数的复合索引，并查询执行：

SYS@ORCL>create index idx_oid2_test_objects on test_objects(object_id,1);

Index created.

SYS@ORCL>select count(*) from test_objects where object_id is null;

COUNT(*)

----------

0

Execution Plan

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

Plan hash value: 1238205220

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

-----------

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

Time |

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

-----------

| 0 | SELECT STATEMENT | | 1 | 5 | 2 (0)|

00:00:01 |

| 1 | SORT AGGREGATE | | 1 | 5 | |

|

|* 2 | INDEX RANGE SCAN| IDX_OID2_TEST_OBJECTS | 1 | 5 | 2 (0)|

00:00:01 |

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

-----------

Predicate Information (identified by operation id):

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

2 - access("OBJECT_ID" IS NULL)

Statistics

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

1 recursive calls

0 db block gets

3 consistent gets

2 physical reads

0 redo size

410 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed
可以看到此时拥有含场数的复合索引之后就能够使用该索引查找null，极大的提升了执行效率。consistent gets从74808降到了3.效果非常明显。