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通常情况下 is null 或者!= 这些条件如果不是具有很强的过滤性，可以先关注其它的过滤条件。但有些 SQL 这两种条件具有很强的过滤性，就可以考虑用以下方法。下面先讨论 is null 的优化，再讨论!= 的优化，最后讨论 is null or != 一起使用的优化。
以下测试：
Oracle version:11.2.0.4
# 新建测试表
create table scott.tb_sj01 as select * from dba_objects;
# 处理测试表中的数据
update scott.tb_sj01 set object_name=null where object_id<=10;
update scott.tb_sj01 set object_name=’SJ’ where object_id>20;
commit;
# 收集表的统计信息
begin
dbms_stats.gather_table_stats(‘scott’,’TB_SJ01′);
end;
# 查看测试表中 object_name 字段的数据分布
# 可以看出 object_name 字段 IS NULL 的有 9 笔记录;object_name !=’SJ’ 有 11 笔记录。
select nvl(object_name,’NULL’) object_name,count(1) cnt
from scott.tb_sj01
group by nvl(object_name,’NULL’)
order by 2 ;
/*
OBJECT_NAME CNT
ICOL$ 1
TS$ 1
OBJ$ 1
FILE$ 1
STDBY_LINK_CT6601SB 1
UNDO$ 1
FET$ 1
I_USER# 1
UET$ 1
SEG$ 1
IND$ 1
NULL 9
SJ 86880
*/
1. 语句：select * from scott.tb_sj01 where object_name is null 的优化
SQL> set autot trace
# 优化前,COST=346,consistent gets=1246
SQL> select * from scott.tb_sj01 where object_name is null;
9 rows selected.
Execution Plan
———————————————————-
Plan hash value: 283620643
—————————————————————————–
| Id | Operation | Name | Rows | Bytes| Cost (%CPU)| Time |
—————————————————————————–
| 0 | SELECT STATEMENT | | 9 | 684 | 346 (1)| 00:00:05 |
|* 1 | TABLE ACCESS FULL| TB_SJ01 | 9 | 684 | 346 (1)| 00:00:05 |
—————————————————————————–
Predicate Information (identified by operation id):
—————————————————
1 – filter(“OBJECT_NAME” IS NULL)
Statistics
———————————————————-
0 recursive calls
0 db block gets
1246 consistent gets
0 physical reads
0 redo size
1850 bytes sent via SQL*Net to client
523 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
9 rows processed
# 优化方法 1:
# 增加索引，让 object_name IS NULL 的也保存在索引中
SQL> create index scott.idx_tb_sj01_01 on scott.tb_sj01(object_name,1);
# 优化后，COST=3,consistent gets=5
SQL> select * from scott.tb_sj01 where object_name is null;
9 rows selected.
Execution Plan
———————————————————-
Plan hash value: 1042936765
———————————————————————————————-
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
———————————————————————————————-
| 0 | SELECT STATEMENT | | 9 | 684 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TB_SJ01 | 9 | 684 | 3 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_TB_SJ01_01 | 9 | | 2 (0)| 00:00:01 |
———————————————————————————————-
Predicate Information (identified by operation id):
—————————————————
2 – access(“OBJECT_NAME” IS NULL)
Statistics
———————————————————-
0 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
1850 bytes sent via SQL*Net to client
523 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
9 rows processed
# 优化方法 2:
# 增加函数索引，只让 object_name IS NULL 的保存在索引中
create index scott.idx_tb_sj01_02 on scott.tb_sj01(decode(object_name,null,1));
# 原语句改写为：
select * from scott.tb_sj01 where decode(object_name,null,1)=1;
# 优化后，COST=2，consistent gets=4
SQL> select * from scott.tb_sj01 where decode(object_name,null,1)=1;
9 rows selected.
Execution Plan
———————————————————-
Plan hash value: 612345449
———————————————————————————————-
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
———————————————————————————————-
| 0 | SELECT STATEMENT | | 869 | 66044 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TB_SJ01 | 869 | 66044 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_TB_SJ01_02 | 9 | | 1 (0)| 00:00:01 |
———————————————————————————————-
Predicate Information (identified by operation id):
—————————————————
2 – access(DECODE(“OBJECT_NAME”,NULL,1)=1)
Statistics
———————————————————-
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
1850 bytes sent via SQL*Net to client
523 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
9 rows processed
2. 语句:select * from scott.tb_sj01 where object_name!=’SJ’ 的优化
# 优化前,COST=346，consistent gets=1246
SQL> select * from scott.tb_sj01 where object_name!=’SJ’;
11 rows selected.
Execution Plan
———————————————————-
Plan hash value: 283620643
—————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
—————————————————————————–
| 0 | SELECT STATEMENT | | 79650 | 5911K| 346 (1)| 00:00:05 |
|* 1 | TABLE ACCESS FULL| TB_SJ01 | 79650 | 5911K| 346 (1)| 00:00:05 |
—————————————————————————–
Predicate Information (identified by operation id):
—————————————————
1 – filter(“OBJECT_NAME”<>’SJ’)
Statistics
———————————————————-
1 recursive calls
0 db block gets
1246 consistent gets
0 physical reads
0 redo size
1979 bytes sent via SQL*Net to client
523 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
11 rows processed
# 优化方法 1：
# 增加函数索引，将 object_name!=’SJ’ 和 object_name IS NULL 的保存在索引中
create index scott.idx_tb_sj01_04 on scott.tb_sj01(decode(object_name,null,1,’SJ’,null,2));
drop index scott.idx_tb_sj01_04;
# 原语句改写为：
select * from scott.tb_sj01 t where decode(object_name,null,1,’SJ’,null,2)=2;
# 优化后，COST=4，consistent gets=4
SQL> select * from scott.tb_sj01 t where decode(object_name,null,1,’SJ’,null,2)=2;
11 rows selected.
Execution Plan
———————————————————-
Plan hash value: 3453712045
———————————————————————————————-
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
———————————————————————————————-
| 0 | SELECT STATEMENT | | 869 | 71258 | 4 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TB_SJ01 | 869 | 71258 | 4 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_TB_SJ01_04 | 20 | | 1 (0)| 00:00:01 |
———————————————————————————————-
Predicate Information (identified by operation id):
—————————————————
2 – access(DECODE(“OBJECT_NAME”,NULL,1,’SJ’,NULL,2)=2)
Statistics
———————————————————-
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
1938 bytes sent via SQL*Net to client
523 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
11 rows processed
# 优化方法 2：
# 增加函数索引，只让 object_name!=’SJ’ 的保存在索引中
create index scott.idx_tb_sj01_05 on scott.tb_sj01(case when object_name!=’SJ’ then 1 else null end);
# 原语句改写为：
select * from scott.tb_sj01 t where (case when object_name!=’SJ’ then 1 else null end)=1;
# 优化后，COST=3，consistent gets=4
SQL> select * from scott.tb_sj01 t where (case when object_name!=’SJ’ then 1 else null end)=1;
11 rows selected.
Execution Plan
———————————————————-
Plan hash value: 376302892
———————————————————————————————-
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
———————————————————————————————-
| 0 | SELECT STATEMENT | | 869 | 71258 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TB_SJ01 | 869 | 71258 | 3 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_TB_SJ01_05 | 11 | | 1 (0)| 00:00:01 |
———————————————————————————————-
Predicate Information (identified by operation id):
—————————————————
2 – access(CASE WHEN “OBJECT_NAME”<>’SJ’ THEN 1 ELSE NULL END =1)
Statistics
———————————————————-
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
1938 bytes sent via SQL*Net to client
523 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
11 rows processed
3. 语句：select * from scott.tb_sj01 where object_name is null or object_name !=’SJ’ 的优化
# 优化前,COST=346，consistent gets=1247
SQL> select * from scott.tb_sj01 where object_name is null or object_name !=’SJ’;
20 rows selected.
Execution Plan
———————————————————-
Plan hash value: 283620643
—————————————————————————–
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
—————————————————————————–
| 0 | SELECT STATEMENT | | 40 | 3280 | 346 (1)| 00:00:05 |
|* 1 | TABLE ACCESS FULL| TB_SJ01 | 40 | 3280 | 346 (1)| 00:00:05 |
—————————————————————————–
Predicate Information (identified by operation id):
—————————————————
1 – filter(“OBJECT_NAME”<>’SJ’ OR “OBJECT_NAME” IS NULL)
Statistics
———————————————————-
0 recursive calls
0 db block gets
1247 consistent gets
0 physical reads
0 redo size
2403 bytes sent via SQL*Net to client
534 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
20 rows processed
# 优化方法 1：
# 增加索引，将 object_name!=’SJ’ 和 object_name IS NULL 的保存在索引中
create index scott.idx_tb_sj01_06 on scott.tb_sj01(decode(object_name,null,1,’SJ’,null,1));
# 语句修改为：
select * from scott.tb_sj01 t where decode(object_name,null,1,’SJ’,null,1)=1;
# 优化后,COST=3，consistent gets=6
SQL> select * from scott.tb_sj01 where decode(object_name,null,1,’SJ’,null,1)=1;
20 rows selected.
Execution Plan
———————————————————-
Plan hash value: 356892721
———————————————————————————————-
| Id | Operation | Name | Rows | Bytes | Cost (%CP
U)| Time |
———————————————————————————————-
| 0 | SELECT STATEMENT | | 869 | 71258 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TB_SJ01 | 869 | 71258 | 3 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_TB_SJ01_06 | 20 | | 1 (0)| 00:00:01 |
———————————————————————————————-
Predicate Information (identified by operation id):
—————————————————
2 – access(DECODE(“OBJECT_NAME”,NULL,1,’SJ’,NULL,1)=1)
Statistics
———————————————————-
0 recursive calls
0 db block gets
6 consistent gets
0 physical reads
0 redo size
2362 bytes sent via SQL*Net to client
534 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
20 rows processed
SQL> set autot off
# 优化方法 2，增加 case when 的函数索引，改写语句，同语句 2，略
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