使用索引的誤區之六：爲所有列都建立索引
我們知道，建立索引是爲了提高查詢的效率，但是同時也應該注意到，索引增加了對DML操作（insert, update, delete）的代價，而且，一給中的索引如果太多，那麼多數的索引是根本不會被使用到的，而另一方面我們維護這些不被使用的所以還要大幅度降低系統的性能。所以，索引不是越多越好，而是要恰到好處的使用。
比如說，有些列由於使用了函數，我們要使用已有的索引（如一些複合索引）是不可能的（詳細請參見前面“函數索引”），那麼就必須建立單獨的函數索引，如果說這個函數索引很少會被應用（僅僅在幾個特別的sql中會用到），我們就可以嘗試改寫查詢，而不去建立和維護那個函數索引，例如：
C:/>sqlplus demo/demo
SQL*Plus: Release 9.2.0.1.0 - Production on Sun Oct 17 07:47:30 2004
Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.
Connected to:
Oracle9i Enterprise Edition Release 9.2.0.1.0 - Production
With the OLAP and Oracle Data Mining options
JServer Release 9.2.0.1.0 - Production
SQL> show user
User is "demo"
SQL> desc emp
Name Type Nullable Default Comments
-------- ------------ -------- ------- --------
EMPNO NUMBER(4)
ENAME VARCHAR2(10) Y
JOB VARCHAR2(9) Y
MGR NUMBER(4) Y
HIREDATE DATE Y
SAL NUMBER(7,2) Y
COMM NUMBER(7,2) Y
DEPTNO NUMBER(2) Y
SQL> create index emp_id3 on emp(hiredate);
Index created
1，trunc函數
SQL> select empno,ename,deptno from emp where trunc(hiredate)=@#2004-01-01@#;
no rows selected
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 TABLE ACCESS (FULL) OF @#EMP@#
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
328 bytes sent via SQL*Net to client
372 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
將上面的查詢轉換爲：
SQL> select empno,ename,deptno from emp
2 where hiredate >= to_date(@#2004-01-01@#,@#yyyy-mm-dd@#)
3 and hiredate<to_date(@#2004-01-01@#,@#yyyy-mm-dd@#)+0.999;
no rows selected
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 TABLE ACCESS (BY INDEX ROWID) OF @#EMP@#
2 1 INDEX (RANGE SCAN) OF @#EMP_ID3@# (NON-UNIQUE)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
1 consistent gets
0 physical reads
0 redo size
328 bytes sent via SQL*Net to client
372 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
SQL>
2，to_char函數
SQL> select empno,ename,deptno from emp
2 where to_char(hiredate,@#yyyy-mm-dd@#)=@#2003-09-05@#;
no rows selected
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 TABLE ACCESS (FULL) OF @#EMP@#
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
328 bytes sent via SQL*Net to client
372 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
SQL> select empno,ename,deptno from emp
2 where hiredate=to_date(@#2003-09-05@#,@#yyyy-mm-dd@#);
no rows selected
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 TABLE ACCESS (BY INDEX ROWID) OF @#EMP@#
2 1 INDEX (RANGE SCAN) OF @#EMP_ID3@# (NON-UNIQUE)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
1 consistent gets
0 physical reads
0 redo size
328 bytes sent via SQL*Net to client
372 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
SQL>
注意，轉換時注意數據庫中對字段的精度，如@#yyyymmyy@#，或者@#yyyymmddhh24miss@#
3，to_date函數
參見上面的方法
4，substr函數
SQL> desc dept
Name Null? Type
----------------------------------------- -------- ----------------------------
DEPTNO NUMBER(2)
DNAME VARCHAR2(14)
LOC VARCHAR2(13)
SQL> create index dept_id1 on dept(dname);
Index created.
SQL> select dname from dept where substr(dname,1,3)=@#abc@#;
no rows selected
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 TABLE ACCESS (FULL) OF @#DEPT@#
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
7 consistent gets
0 physical reads
0 redo size
221 bytes sent via SQL*Net to client
372 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
SQL> select dname from dept where dname like @#abc%@#;
no rows selected
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 INDEX (RANGE SCAN) OF @#DEPT_ID1@# (NON-UNIQUE)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
1 consistent gets
0 physical reads
0 redo size
221 bytes sent via SQL*Net to client
372 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
SQL>
通常，爲了均衡查詢的效率和DML的效率，我們要仔細的分析應用，找出來出現頻率相對較多、字段內容較少（比如varchar2(1000)就不適合建立索引，而varchar2(10)相對來說就適合建立索引）的列，合理的建立索引，比如有時候我們希望建立複合索引，有時候我們更希望建立單鍵索引。
事實上，oracle的索引還有很多話題，如監視索引的使用情況，Oracle 9i 推出的SKIP SCAN等等，在下一個討論中，我們會對這些話題詳細闡述。
使用索引的誤區之六：爲索引列都建立索引
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