一般而言，以下情况都会使Oracle的优化器走全表扫描，举例:
1. substr(hbs_bh,1,4)=’5400’，优化处理：hbs_bh like ‘5400%’
2. trunc(sk_rq)=trunc(sysdate)， 优化处理：sk_rq>=trunc(sysdate) and sk_rq<trunc(sysdate+1)
3. 进行了显式或隐式的运算的字段不能进行索引，如：
ss_df+20>50，优化处理：ss_df>30
'X' || hbs_bh>’X5400021452’，优化处理：hbs_bh>'5400021542'
sk_rq+5=sysdate，优化处理：sk_rq=sysdate-5
4. 条件内包括了多个本表的字段运算时不能进行索引，如：ys_df>cx_df，无法进行优化
qc_bh || kh_bh='5400250000'，优化处理：qc_bh='5400' and kh_bh='250000'
5. 避免出现隐式类型转化
hbs_bh=5401002554，优化处理：hbs_bh='5401002554'，注：此条件对hbs_bh 进行隐式的to_number转换，因为hbs_bh字段是字符型。
有一些其它的例外情况，如果select 后边只有索引列且where查询中的索引列含有非空约束的时候，以上规则不适用，如下示例：
先给出所有脚本及结论：
drop table t purge;
Create Table t nologging As select * from dba_objects d ;
create index ind_objectname on t(object_name);
select t.object_name from t where t.object_name ='T'; --走索引
select t.object_name from t where UPPER(t.object_name) ='T'; --不走索引
select t.object_name from t where UPPER(t.object_name) ='T' and t.object_name IS NOT NULL ; --走索引 (INDEX FAST FULL SCAN)
select t.object_name from t where UPPER(t.object_name) ||'AAA' ='T'||'AAA' and t.object_name IS NOT NULL ; --走索引 (INDEX FAST FULL SCAN)
select t.object_name,t.owner from t where UPPER(t.object_name) ||'AAA' ='T'||'AAA' and t.object_name IS NOT NULL ; --不走索引
测试代码：
|
C:\Users\华荣>sqlplus lhr/lhr@orclasm
SQL*Plus: Release 11.2.0.1.0 Production on 星期三 11月 12 10:52:29 2014
Copyright (c) 1982, 2010, Oracle. All rights reserved.
连接到:
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
With the Partitioning, Automatic Storage Management, OLAP, Data Mining
and Real Application Testing options
SQL>
SQL>
SQL> drop table t purge;
表已删除。
SQL> Create Table t nologging As select * from dba_objects d ;
表已创建。
SQL> create index ind_objectname on t(object_name);
索引已创建。
|
---- t表所有列均可以为空
|
SQL> desc t
Name Null? Type
----------------------------------------- -------- ----------------------------
OWNER VARCHAR2(30)
OBJECT_NAME VARCHAR2(128)
SUBOBJECT_NAME VARCHAR2(30)
OBJECT_ID NUMBER
DATA_OBJECT_ID NUMBER
OBJECT_TYPE VARCHAR2(19)
CREATED DATE
LAST_DDL_TIME DATE
TIMESTAMP VARCHAR2(19)
STATUS VARCHAR2(7)
TEMPORARY VARCHAR2(1)
GENERATED VARCHAR2(1)
SECONDARY VARCHAR2(1)
NAMESPACE NUMBER
EDITION_NAME VARCHAR2(30)
SQL>
SQL> set autotrace traceonly;
SQL> select t.object_name from t where t.object_name ='T';
执行计划
----------------------------------------------------------
Plan hash value: 4280870634
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 66 | 3 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| IND_OBJECTNAME | 1 | 66 | 3 (0)| 00:00:01 |
-----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T"."OBJECT_NAME"='T')
Note
-----
- dynamic sampling used for this statement (level=2)
- SQL plan baseline "SQL_PLAN_503ygb00mbj6k165e82cd" used for this statement
统计信息
----------------------------------------------------------
34 recursive calls
43 db block gets
127 consistent gets
398 physical reads
15476 redo size
349 bytes sent via SQL*Net to client
359 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select t.object_name from t where UPPER(t.object_name) ='T';
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 12 | 792 | 305 (1)| 00:00:04 |
|* 1 | TABLE ACCESS FULL| T | 12 | 792 | 305 (1)| 00:00:04 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(UPPER("T"."OBJECT_NAME")='T')
Note
-----
- dynamic sampling used for this statement (level=2)
- SQL plan baseline "SQL_PLAN_9p76pys5gdb2b94ecae5c" used for this statement
统计信息
----------------------------------------------------------
29 recursive calls
43 db block gets
1209 consistent gets
1092 physical reads
15484 redo size
349 bytes sent via SQL*Net to client
359 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select t.object_name from t where UPPER(t.object_name) ='T' and t.object_name IS NOT NULL ;
执行计划
----------------------------------------------------------
Plan hash value: 3379870158
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 51 | 3366 | 110 (1)| 00:00:02 |
|* 1 | INDEX FAST FULL SCAN| IND_OBJECTNAME | 51 | 3366 | 110 (1)| 00:00:02 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("T"."OBJECT_NAME" IS NOT NULL AND UPPER("T"."OBJECT_NAME")='T')
Note
-----
- dynamic sampling used for this statement (level=2)
- SQL plan baseline "SQL_PLAN_czkarb71kthws18b0c28f" used for this statement
统计信息
----------------------------------------------------------
29 recursive calls
43 db block gets
505 consistent gets
384 physical reads
15612 redo size
349 bytes sent via SQL*Net to client
359 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select t.object_name,t.owner from t where UPPER(t.object_name) ||'AAA' ='T'||'AAA' and t.object_name IS NOT NULL ;
执行计划
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 51 | 4233 | 304 (1)| 00:00:04 |
|* 1 | TABLE ACCESS FULL| T | 51 | 4233 | 304 (1)| 00:00:04 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("T"."OBJECT_NAME" IS NOT NULL AND
UPPER("T"."OBJECT_NAME")||'AAA'='TAAA')
Note
-----
- dynamic sampling used for this statement (level=2)
- SQL plan baseline "SQL_PLAN_au9a1c4hwdtb894ecae5c" used for this statement
统计信息
----------------------------------------------------------
30 recursive calls
44 db block gets
1210 consistent gets
1091 physical reads
15748 redo size
408 bytes sent via SQL*Net to client
359 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> select t.object_name from t where UPPER(t.object_name) ||'AAA' ='T'||'AAA' and t.object_name IS NOT NULL ;
执行计划
----------------------------------------------------------
Plan hash value: 3379870158
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 51 | 3366 | 110 (1)| 00:00:02 |
|* 1 | INDEX FAST FULL SCAN| IND_OBJECTNAME | 51 | 3366 | 110 (1)| 00:00:02 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("T"."OBJECT_NAME" IS NOT NULL AND
UPPER("T"."OBJECT_NAME")||'AAA'='TAAA')
Note
-----
- dynamic sampling used for this statement (level=2)
- SQL plan baseline "SQL_PLAN_1gu36rnh3s2a318b0c28f" used for this statement
统计信息
----------------------------------------------------------
28 recursive calls
44 db block gets
505 consistent gets
6 physical reads
15544 redo size
349 bytes sent via SQL*Net to client
359 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL>
|
其实很好理解的，索引可以看成是小表，一般而言索引总是比表本身要小得多，如果select 后需要检索的项目在索引中就可以检索的到那么Oracle优化器为啥还去大表中寻找数据呢？
最新内容请见作者的GitHub页：http://qaseven.github.io/
