本文介绍了几种不使用索引的情况，本文实验的Oracle数据库版本均为11.2.0.4
情况1：
我们在使用一个B*树索引，而且谓词中没有使用索引的最前列。
如果这种情况，可以假设有一个表T，在T(x,y)上有一个索引。要做以下查询：select * from t where y=5。此时，优化器就不打算使用T(x,y)上的索引，因为谓词中不涉及X列。在这种情况下，倘若使用索引，可能就必须查看每个索引条目，而优化器通常更倾向于对T表做一个全表扫描。
zx@ORCL>create table t as select rownum x,rownum+1 y,rownum+2 z from dual connect by level < 100000;
Table created.
zx@ORCL>select count(*) from t;
COUNT(*)
----------
99999
zx@ORCL>create index idx_t on t(x,y);
Index created.
zx@ORCL>exec dbms_stats.gather_table_stats(user,'T',cascade=>true);
PL/SQL procedure successfully completed.
zx@ORCL>set autotrace traceonly explain
--where条件使用y=5
zx@ORCL>select * from t where y=5;
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 15 | 80 (2)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T | 1 | 15 | 80 (2)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("Y"=5)
--where条件使用x=5
zx@ORCL>select * from t where x=5;
Execution Plan
----------------------------------------------------------
Plan hash value: 1594971208
-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 15 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 15 | 3 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IDX_T | 1 | | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("X"=5)
但这并不完全排除使用索引。如果查询是select x,y from t where y=5，优化器就会注意到，它不必全面扫描表来得到X或Y(x和y都在索引中)，对索引本身做一个民快速的全面扫描会更合适，因为这个索引一般比底层表小得多。还要注意，仅CBO能使用这个访问路径。
zx@ORCL>select x,y from t where y=5;
Execution Plan
----------------------------------------------------------
Plan hash value: 2497555198
------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 10 | 81 (2)| 00:00:01 |
|* 1 | INDEX FAST FULL SCAN| IDX_T | 1 | 10 | 81 (2)| 00:00:01 |
------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("Y"=5)
另一种情况下CBO也会使用T(x,y)上的索引，这就是索引跳跃式扫描。当且仅当索引的最前列(在上面的例子中最前列是x)只有很少的几个不同值，而且优化器了解这一点，跳跃式扫描(skip scan)就能很好地发挥作用。例如，考虑(GEMDER,EMPNO)上的一个索引，其中GENDER可取值有M和F，而且EMPNO是唯一的。对于以下查询:
select * from t where empno=5;
可以考虑使用T上的那个索引采用跳跃式扫描方法来满足这个查询，这说明从概念上讲这个查询会如下处理：
select * from t where GENDER='M' and empno=5
union all
select * from t where GENDER='F' and empno=5
它会跳跃式地扫描索引，以为这是两个索引：一个对应值M，另一个对应值F。
zx@ORCL>create table t1 as select decode(mod(rownum,2),0,'M','F') gender,all_objects.* from all_objects;
Table created.
zx@ORCL>create index idx_t1 on t1(gender,object_id);
Index created.
zx@ORCL>exec dbms_stats.gather_table_stats(user,'T1',cascade=>true);
PL/SQL procedure successfully completed.
zx@ORCL>set autotrace traceonly explain
zx@ORCL>select * from t1 where object_id=42;
Execution Plan
----------------------------------------------------------
Plan hash value: 4072187533
-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 100 | 4 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 100 | 4 (0)| 00:00:01 |
|* 2 | INDEX SKIP SCAN | IDX_T1 | 1 | | 3 (0)| 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID"=42)
filter("OBJECT_ID"=42)
INDEX SKIP SCAN 步骤告诉Oralce要跳跃式扫描这个索引，查询GENDER值有改变的地方，并从那里开始向下读树，然后在所考虑的各个虚拟索引中查询OBJECT_id=42。如果大幅增加GENDER的可取值，如下：
zx@ORCL>alter table t1 modify GENDER varchar2(2);
Table altered.
zx@ORCL>update t1 set gender=(chr(mod(rownum,1024)));
84656 rows updated.
zx@ORCL>commit;
Commit complete.
zx@ORCL>exec dbms_stats.gather_table_stats(user,'T1',cascade=>true);
PL/SQL procedure successfully completed.
zx@ORCL>set autotrace traceonly explain
zx@ORCL>select * from t1 where object_id=42;
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 101 | 344 (1)| 00:00:05 |
|* 1 | TABLE ACCESS FULL| T1 | 1 | 101 | 344 (1)| 00:00:05 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_ID"=42)
情况2：
在使用select count(*) from t查询(或类似的查询)，而且在表T上有一个B*树索引。不过，优化器并不是统计索引条目，而是在全面扫描这个表(尽管索引比表要小)。在这种情况下，索引可能建立在一个允许有NULL值的列上。由于对于索引键完全为null的行不会建立相应的索引条目，所以索引中的行数可能并不是表中的行数。这里优化器的选择是对的，如若不然，倘若它使用索引来统计行数，则可能会得到一个错误的答案。
zx@ORCL>desc t;
Name Null? Type
----------------------------------------------------------------------------------------------------- -------- --------------------------------------------------------------------
X NUMBER
Y NUMBER
Z CHAR(23)
zx@ORCL>select count(*) from t;
Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 153 (1)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T | 99999 | 153 (1)| 00:00:02 |
-------------------------------------------------------------------
zx@ORCL>alter table t modify y not null;
Table altered.
zx@ORCL>desc t
Name Null? Type
----------------------------------------------------------------------------------------------------- -------- --------------------------------------------------------------------
X NUMBER
Y NOT NULL NUMBER
Z CHAR(23)
zx@ORCL>select count(*) from t;
Execution Plan
----------------------------------------------------------
Plan hash value: 2371838348
-----------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-----------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 80 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FAST FULL SCAN| IDX_T | 99999 | 80 (0)| 00:00:01 |
-----------------------------------------------------------------------
情况3：
对于一个有索引的列，做以下查询：
select * from t where function(indexed_column)=value;
却发现没有使用indexed_colum上的索引。原因是这个列上使用了函数。如果是对indexed_column的值建立了索引，而不是对function(indexed_column)的值建索引。在此不能使用这个索引。如果愿意，可以另外对函数建立索引。
zx@ORCL>select * from t where mod(x,999)=1;
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1000 | 34000 | 153 (1)| 00:00:02 |
|* 1 | TABLE ACCESS FULL| T | 1000 | 34000 | 153 (1)| 00:00:02 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(MOD("X",999)=1)
zx@ORCL>create index idx_t_f on t(mod(x,999));
Index created.
zx@ORCL>exec dbms_stats.gather_table_stats(USER,'T',cascade=>true);
PL/SQL procedure successfully completed.
zx@ORCL>select * from t where mod(x,999)=1;
Execution Plan
----------------------------------------------------------
Plan hash value: 4125918735
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 100 | 3800 | 102 (0)| 00:00:02 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 100 | 3800 | 102 (0)| 00:00:02 |
|* 2 | INDEX RANGE SCAN | IDX_T_F | 100 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access(MOD("X",999)=1)
情况4：
考虑以下情况，已经对一个字符钱建立了索引。这个列只包含数据数据。如果使用以下语法来查询：
select * from t where indexed_colum=5;
注意查询中的数字5是常数5(而不是一个字符串)，此时就没有使用INDEXED_COLUMN上的索引。这是因为，前面的查询等价于以下查询：
select * from t where to_number(indexed_column)=5;
我们对这个列隐式地应用了一个函数，如情况3所述，这就会禁止使用这个索引。
zx@ORCL>create table t2 (x char(1) constraint t2_pk primary key ,y date);
Table created.
zx@ORCL>insert into t2 values('5',sysdate);
1 row created.
zx@ORCL>commit;
Commit complete.
zx@ORCL>exec dbms_stats.gather_table_stats(USER,'T2',cascade=>true);
PL/SQL procedure successfully completed.
zx@ORCL>explain plan for select * from t2 where x=5;
Explained.
zx@ORCL>select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 1513984157
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 12 | 3 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| T2 | 1 | 12 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(TO_NUMBER("X")=5)
Note
-----
- dynamic sampling used for this statement (level=2)
可以看到，它会全面扫描表；另外即使我们对查询给出了以下提示：
zx@ORCL>explain plan for select /*+ index(t2 t2_pk) */ * from t2 where x=5;
Explained.
zx@ORCL>select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 3365102699
-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 10 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T2 | 1 | 10 | 2 (0)| 00:00:01 |
|* 2 | INDEX FULL SCAN | T2_PK | 1 | | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(TO_NUMBER("X")=5)
在此使用了索引，但是并不像我们想像中那样对索引完成唯一扫描(UNIQUE SCAN)，而是完成了全面扫描(FULL SCAN)。原因从最后一行输出可以看出:filter(TO_NUMBER("X")=5)。这里对这个数据库列应用了一个隐式函数。X中存储的字符串必须转换为一个数字，之后才能与值5进行比较。在此无法把5转换为一个串，因为我们的NLS(国家语言支持)设置会控制5转换成串时的具体形式(而这是不确定的，不同的NLS设置会有不同的控制)，所以应当把串转为数据。而这样一样(由于应用也函数)，就无法使用索引来快速地查找这一行了。如果只是执行串与串的比较：
zx@ORCL>explain plan for select * from t2 where x='5';
Explained.
zx@ORCL>select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 3897349516
-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 12 | 1 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T2 | 1 | 12 | 1 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | T2_PK | 1 | | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("X"='5')
14 rows selected.
不出所料，这会得到我们期望的INDEX UNIQUE SCAN,而且可以看到这里没有应用函数。一定要尽可能地避免隐式转换。
还经常出现一个关于日期的问题，如果做以下查询：
select * from t where trunc(date_col)=trunc(sysdate);
而且发现这个查询没有使用DATE_COL上的索引，为了解决这个问题，可以对trunc(date_col)建立索引，或者使用区间比较运算符来查询(也许这是更容易的做法)。下面来看对日期使用大于或小于运算符的一个例子。可以认识到以下条件：
trunc(date_col)=trunc(sysdate)
与下面的条件是一样的：
date_col>= trunc(sysdate) and date_colcreate table t3 (x,y null,primary key (x) ) as select rownum x,object_name y from all_objects;
Table created.
zx@ORCL>exec dbms_stats.gather_table_stats(USER,'T3',cascade=>true);
PL/SQL procedure successfully completed.
zx@ORCL>set autotrace traceonly explain
--运行一个查询查询相对较少的数据
zx@ORCL>select count(y) from t3 where x