SQL条件中“is null”谓词导致全表扫描问题优化

SQL条件中“is null”谓词导致全表扫描问题优化

摘要：SQL，索引，is null，优化

正文：

前段时间针对相关SQL进行了优化，零散记录了下优化过程，这次整理出来与大家分享。若有描述不当或补充的地方请留言，谢谢！

方法1：用固定值建复合索引优化

1）新建测试表
create table hr_tab1(id number(10) not null,name varchar2(20) );

2）未建索引之前执行计划
SQL>explain plan for select * from hr_tab1 where name is null;

Plan hash value: 2713387046

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

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

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

| 0 | SELECT STATEMENT | | 1 | 25 | 2 (0)| 00:00:01 |

|* 1 | TABLE ACCESS FULL| HR_TAB1 | 1 | 25 | 2 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 - filter("NAME" IS NULL)

Note

-----

- 'PLAN_TABLE' is old version

- dynamic sampling used for this statement

3）使用空值列name跟一个固定值建复合索引
建复合索引：
create index IND_HR_TAB1_HH1 on hr_tab1 (name,1)

4）查看优化后执行计划

执行SQL：
SQL>explain plan for select * from hr_tab1 where name is null;

Plan hash value: 538103671

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

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

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

| 0 | SELECT STATEMENT | | 1 | 25 | 0 (0)| 00:00:01 |

| 1 | TABLE ACCESS BY INDEX ROWID| HR_TAB1 | 1 | 25 | 0 (0)| 00:00:01 |

|* 2 | INDEX RANGE SCAN | IND_HR_TAB1_HH1 | 1 | | 0 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

2 - access("NAME" IS NULL)

Note

-----

- 'PLAN_TABLE' is old version

- dynamic sampling used for this statement (level=2)

从执行计划上看，建了复合索引之后，在查询is null时走了范围扫描索引。

在这里有必要说明一下为什么会走索引范围扫描。索引范围扫描主要出现在以下些情况：
在唯一索引上：在唯一索引上使用一个索引存取多行数据，

这种情况索引范围扫描的典型情况是在谓词(where限制条件)中使用了范围 操作符(如>、<、<>、>=、<=、between)
在非唯一索引上：当查询结果可能会返回多行数据时，所以会使用索引范围扫描。

执行SQL：explain plan for select name from hr_tab1 where name is null;

Plan hash value: 3977038940

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

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

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

| 0 | SELECT STATEMENT | | 1 | 12 | 1 (0)| 00:00:01 |

|* 1 | INDEX RANGE SCAN| IND_HR_TAB1_HH1 | 1 | 12 | 1 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 - access("NAME" IS NULL)

Note

-----

- 'PLAN_TABLE' is old version

- dynamic sampling used for this statement (level=2)

方法二：通过函数索引优化
以下是流程待办任务查询SQL，涉及到权限和流程互斥，所以这个SQL比较复杂。
查看优化前执行计划：

SQL> explain plan for select DECODE(NVL(OPERID, '0'), '0', '岗位待办', '个人待办') TASK,
2 PROCESS_DEF_NAME,
3 DECODE(IS_RETURN,
4 '1',
5 '<span style="color:red">' || BAE007 ||
6 '</span>',
7 BAE007) BAE007,
8 ACTION_DEF_NAME,
9 PROCESS_KEY_INFO,
10 '<a href=Workflow.do?pid=' || BAE007 || 'wid=' ||
11 WORK_ITEM_ID || '>办理</a>' BL,
12 DECODE(NVL(OPERID, '0'),
13 '0',
14 '<a href="javascript:jieShou(''' || WORK_ITEM_ID ||
15 ''')">接收</a>',
16 '<a href="javascript:cheXiao(''' || WORK_ITEM_ID ||
17 ''')">撤销</a>') DO,
18 '<a href="javascript:detail(''' || BAE007 ||
19 ''')">经办详情</a>' DETAIL,
20 BSTART_TIME
21 from V_WF_TODOLIST
22 where (OPERID = '114627' or
23 (OPERID is null and BAE006 like '441905%' and exists
24 (select 1
25 from FW_OPERATOR2RIGHT A, FW_RIGHT B
26 where A.RIGHTID = B.RIGHTID
27 and A.AAE100 = B.AAE100
28 and A.AUTHTYPE = '1'
29 and A.AAE100 = '1'
30 and B.ACTION_DEF_ID = V_WF_TODOLIST.ACTION_DEF_ID
31 and A.OPERID = '114627') and not exists
32 (select 1
33 from WF_WORK_ITEM
34 where BAE007 = V_WF_TODOLIST.BAE007
35 and STATE = '5'
36 and FILTER_OPR = OPERID
37 and ACTION_DEF_ID in
38 (select ACTION_DEF_ID
39 from WF_WORK_ITEM
40 where BAE007 = V_WF_TODOLIST.BAE007
41 and LOCATE(ACTION_DEF_ID,
42 V_WF_TODOLIST.X_ACTION_DEF_IDS) > 0)
43 and OPERID = '114627')))
44 order by TASK desc, BSTART_TIME desc, BAE007 desc;

Explained

查看计划描述：
SQL> select * from table (dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 4153392385
--------------------------------------------------------------------------------
| Id | Operation | Name | Row
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1
| 1 | SORT ORDER BY | | 1
| 2 | CONCATENATION | |
|* 3 | FILTER | |
|* 4 | HASH JOIN | | 1
|* 5 | TABLE ACCESS FULL | WF_WORK_ITEM | 1
| 6 | TABLE ACCESS FULL | WF_PROCESS_INSTANCE | 675
|* 7 | TABLE ACCESS BY INDEX ROWID | FW_OPERATOR2RIGHT |
| 8 | NESTED LOOPS | |
|* 9 | TABLE ACCESS BY INDEX ROWID | FW_RIGHT |
|* 10 | INDEX RANGE SCAN | IND_FW_RIGHT_ACTION_DEF_ID |
| 11 | BITMAP CONVERSION TO ROWIDS | |
| 12 | BITMAP AND | |
| 13 | BITMAP CONVERSION FROM ROWIDS | |
|* 14 | INDEX RANGE SCAN | IND_OPERATOR2RIGHT_OPERID | 1

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
| 15 | BITMAP CONVERSION FROM ROWIDS | |
|* 16 | INDEX RANGE SCAN | IND_OPERATOR2RIGHT_RIGHTID | 1
|* 17 | TABLE ACCESS BY INDEX ROWID | WF_WORK_ITEM |
| 18 | NESTED LOOPS | |
|* 19 | TABLE ACCESS BY INDEX ROWID | WF_WORK_ITEM |
|* 20 | INDEX RANGE SCAN | IND_WF_WORK_ITEM |
|* 21 | INDEX RANGE SCAN | IND_WF_WORK_ITEM |
|* 22 | FILTER | |
| 23 | NESTED LOOPS | |
|* 24 | TABLE ACCESS BY INDEX ROWID | WF_WORK_ITEM |
|* 25 | INDEX RANGE SCAN | IND_WF_WORK_ITEM_OPERID |
| 26 | TABLE ACCESS BY INDEX ROWID | WF_PROCESS_INSTANCE |
|* 27 | INDEX UNIQUE SCAN | PK_WF_PROCESS_INSTANCE |
|* 28 | TABLE ACCESS BY INDEX ROWID | WF_WORK_ITEM |
| 29 | NESTED LOOPS | |
|* 30 | TABLE ACCESS BY INDEX ROWID | WF_WORK_ITEM |
|* 31 | INDEX RANGE SCAN | IND_WF_WORK_ITEM |
|* 32 | INDEX RANGE SCAN | IND_WF_WORK_ITEM |
|* 33 | TABLE ACCESS BY INDEX ROWID | FW_OPERATOR2RIGHT |
| 34 | NESTED LOOPS | |
|* 35 | TABLE ACCESS BY INDEX ROWID | FW_RIGHT |

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
|* 36 | INDEX RANGE SCAN | IND_FW_RIGHT_ACTION_DEF_ID |
| 37 | BITMAP CONVERSION TO ROWIDS | |
| 38 | BITMAP AND | |
| 39 | BITMAP CONVERSION FROM ROWIDS| |
|* 40 | INDEX RANGE SCAN | IND_OPERATOR2RIGHT_OPERID | 1
| 41 | BITMAP CONVERSION FROM ROWIDS| |
|* 42 | INDEX RANGE SCAN | IND_OPERATOR2RIGHT_RIGHTID | 1
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter( EXISTS (SELECT 0 FROM "FW_RIGHT" "B","FW_OPERATOR2RIGHT" "A" WHER
"A"."RIGHTID"="B"."RIGHTID" AND "A"."AUTHTYPE"='1' AND "A"."AAE100
"B"."AAE100"='1') AND NOT EXISTS (SELECT 0 FROM "WF_WORK_ITEM" "W
"WF_WORK_ITEM" WHERE "BAE007"=:B2 AND TO_NUMBER("LOCATE"("ACTION_D
AND "ACTION_DEF_ID"="ACTION_DEF_ID" AND "BAE007"=:B5 AND "OPERID"=
"STATE"='5'))
4 - access("B"."BAE007"="A"."BAE007")
5 - filter("A"."OPERID" IS NULL AND "A"."BAE006" LIKE '441905%' AND ("A"."STA
7 - filter("A"."AUTHTYPE"='1' AND "A"."AAE100"='1')

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
9 - filter("B"."AAE100"='1')
10 - access("B"."ACTION_DEF_ID"=:B1)
14 - access("A"."OPERID"='114627')
16 - access("A"."RIGHTID"="B"."RIGHTID")
17 - filter(TO_NUMBER("LOCATE"("ACTION_DEF_ID","FUN_FW_X_ACTION_IDS"(:B1,:B2))
"ACTION_DEF_ID"="ACTION_DEF_ID")
19 - filter("OPERID"='114627' AND "FILTER_OPR"='114627' AND "STATE"='5')
20 - access("BAE007"=:B1)
21 - access("BAE007"=:B1)
22 - filter(LNNVL("A"."OPERID" IS NULL) OR LNNVL("A"."BAE006" LIKE '441905%')
(SELECT 0 FROM "WF_WORK_ITEM" "WF_WORK_ITEM","WF_WORK_ITEM" "WF_WO
TO_NUMBER("LOCATE"("ACTION_DEF_ID","FUN_FW_X_ACTION_IDS"(:B2,:B3))
AND "BAE007"=:B4 AND "OPERID"='114627' AND "FILTER_OPR"='114627' A
0 FROM "FW_RIGHT" "B","FW_OPERATOR2RIGHT" "A" WHERE "A"."OPERID"='
AND "A"."AUTHTYPE"='1' AND "A"."AAE100"='1' AND "B"."ACTION_DEF_ID
24 - filter("A"."STATE"='0' OR "A"."STATE"='2')
25 - access("A"."OPERID"='114627')
27 - access("B"."BAE007"="A"."BAE007")
28 - filter(TO_NUMBER("LOCATE"("ACTION_DEF_ID","FUN_FW_X_ACTION_IDS"(:B1,:B2))
"ACTION_DEF_ID"="ACTION_DEF_ID")
30 - filter("OPERID"='114627' AND "FILTER_OPR"='114627' AND "STATE"='5')

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
31 - access("BAE007"=:B1)
32 - access("BAE007"=:B1)
33 - filter("A"."AUTHTYPE"='1' AND "A"."AAE100"='1')
35 - filter("B"."AAE100"='1')
36 - access("B"."ACTION_DEF_ID"=:B1)
40 - access("A"."OPERID"='114627')
42 - access("A"."RIGHTID"="B"."RIGHTID")
Note
-----
- 'PLAN_TABLE' is old version

94 rows selected



执行计划显示，索引列OPERID 在使用OPERID is null条件查时导致全表扫描了。

优化：
增加函数索引，并修改语句将or 改成union all。

建立函数索引：
create index IND_WF_WORK_ITEM_HS1 on wf_work_item(nvl(operid,'1'));

优化后SQL语句为：

select DECODE(NVL(OPERID, '0'), '0', '岗位待办', '个人待办') TASK,
PROCESS_DEF_NAME,
DECODE(IS_RETURN,
'1',
'<span style="color:red">' || BAE007 ||
'</span>',
BAE007) BAE007,
ACTION_DEF_NAME,
PROCESS_KEY_INFO,
'<a href=Workflow.do?pid=' || BAE007 || 'wid=' ||
WORK_ITEM_ID || '>办理</a>' BL,
DECODE(NVL(OPERID, '0'),
'0',
'<a href="javascript:jieShou(''' || WORK_ITEM_ID ||
''')">接收</a>',
'<a href="javascript:cheXiao(''' || WORK_ITEM_ID ||
''')">撤销</a>') DO,
'<a href="javascript:detail(''' || BAE007 ||
''')">经办详情</a>' DETAIL,
BSTART_TIME
from V_WF_TODOLIST
where OPERID = '114627'
union all
select DECODE(NVL(OPERID, '0'), '0', '岗位待办', '个人待办') TASK,
PROCESS_DEF_NAME,
DECODE(IS_RETURN,
'1',
'<span style="color:red">' || BAE007 ||
'</span>',
BAE007) BAE007,
ACTION_DEF_NAME,
PROCESS_KEY_INFO,
'<a href=Workflow.do?pid=' || BAE007 || 'wid=' ||
WORK_ITEM_ID || '>办理</a>' BL,
DECODE(NVL(OPERID, '0'),
'0',
'<a href="javascript:jieShou(''' || WORK_ITEM_ID ||
''')">接收</a>',
'<a href="javascript:cheXiao(''' || WORK_ITEM_ID ||
''')">撤销</a>') DO,
'<a href="javascript:detail(''' || BAE007 ||
''')">经办详情</a>' DETAIL,
BSTART_TIME
from V_WF_TODOLIST
where exists
(select 1
from FW_OPERATOR2RIGHT A, FW_RIGHT B
where A.RIGHTID = B.RIGHTID
and A.AAE100 = B.AAE100
and A.AUTHTYPE = '1'
and A.AAE100 = '1'
and B.ACTION_DEF_ID = V_WF_TODOLIST.ACTION_DEF_ID
and A.OPERID = '114627')
and not exists
(select 1
from WF_WORK_ITEM
where BAE007 = V_WF_TODOLIST.BAE007
and STATE = '5'
and FILTER_OPR = OPERID
and ACTION_DEF_ID in
(select ACTION_DEF_ID
from WF_WORK_ITEM
where BAE007 = V_WF_TODOLIST.BAE007
and LOCATE(ACTION_DEF_ID,
V_WF_TODOLIST.X_ACTION_DEF_IDS) > 0)
and OPERID = '114627')
and nvl(OPERID,'1') = '1' and BAE006 like '441905%'

order by TASK desc, BSTART_TIME desc, BAE007 desc

经过以上优化后，不再有全表扫描了。