SQL优化中索引列使用函数之灵异事件

　在SQL优化内容中有一种说法说的是避免在索引列上使用函数、运算等操作，否则Oracle优化器将不使用索引而使用全表扫描，但是也有一些例外的情况，今天我们就来看看该灵异事件。

　　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

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> 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

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

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

| 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

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

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

| 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

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

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

| 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

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

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

| 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

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

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

| 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？

posted on 2014-12-03 13:35 顺其自然EVO 阅读(572) 评论(0) 编辑收藏所属分类: 数据库

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