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：
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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
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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？
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