欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页  >  IT编程

[20180408]那些函数索引适合字段的查询.txt

程序员文章站 2022-04-19 18:25:05
[20180408]那些函数索引适合字段的查询.txt--//一般不主张建立函数索引,往往是开发的无知,使用trunc等函数,实际上一些函数也可以用于字段的查询.--//以前零碎的写过一些,放假看了https://blog.pythian.com/tackling-time-troubles-use ......

[20180408]那些函数索引适合字段的查询.txt

--//一般不主张建立函数索引,往往是开发的无知,使用trunc等函数,实际上一些函数也可以用于字段的查询.
--//以前零碎的写过一些,放假看了https://blog.pythian.com/tackling-time-troubles-use-dates-correctly-oracle/.
--//自己也做一些总结:

1.环境:
SCOTT@test01p> @ ver1
PORT_STRING                    VERSION        BANNER                                                                               CON_ID
------------------------------ -------------- -------------------------------------------------------------------------------- ----------
IBMPC/WIN_NT64-9.1.0           12.1.0.1.0     Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production              0

2.trunc函数:
--//trunc函数是开发最常使用的函数,实际上还是一句话无知,开发根本不知道这样建立查询条件导致普通索引无效,必须建立函数索引.
--//但是建立trunc函数索引却可以使用在普通日期字段的查询.

SCOTT@test01p> create index if_emp_hiredate on emp(trunc(hiredate));
Index created.

SCOTT@test01p> select * from emp where hiredate=to_date('1980/12/17','yyyy-mm-dd');

     EMPNO ENAME      JOB              MGR HIREDATE                   SAL       COMM     DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
      7369 SMITH      CLERK           7902 1980-12-17 00:00:00        800                    20

SCOTT@test01p> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  1ct8dum3uyy7u, child number 0
-------------------------------------
select * from emp where hiredate=to_date('1980/12/17','yyyy-mm-dd')
Plan hash value: 4059437819
--------------------------------------------------------------------------------------------------------
| Id  | Operation                           | Name            | E-Rows |E-Bytes| Cost (%CPU)| E-Time   |
--------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |                 |        |       |     2 (100)|          |
|*  1 |  TABLE ACCESS BY INDEX ROWID BATCHED| EMP             |      1 |    38 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN                  | IF_EMP_HIREDATE |      1 |       |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1 / EMP@SEL$1
   2 - SEL$1 / EMP@SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("HIREDATE"=TO_DATE(' 1980-12-17 00:00:00', 'syyyy-mm-dd hh24:mi:ss'))
   2 - access("EMP"."SYS_NC00010$"=TRUNC(TO_DATE(' 1980-12-17 00:00:00', 'syyyy-mm-dd hh24:mi:ss')))      

--//顺便看看范围查询是否有效:
SCOTT@test01p> select * from emp where hiredate between to_date('1980/12/17','yyyy-mm-dd') and to_date('1980/12/18','yyyy-mm-dd');
     EMPNO ENAME      JOB              MGR HIREDATE                   SAL       COMM     DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
      7369 SMITH      CLERK           7902 1980-12-17 00:00:00        800                    20

SCOTT@test01p> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  a00watu79k28f, child number 0
-------------------------------------
select * from emp where hiredate between
to_date('1980/12/17','yyyy-mm-dd') and
to_date('1980/12/18','yyyy-mm-dd')
Plan hash value: 4059437819
--------------------------------------------------------------------------------------------------------
| Id  | Operation                           | Name            | E-Rows |E-Bytes| Cost (%CPU)| E-Time   |
--------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |                 |        |       |     2 (100)|          |
|*  1 |  TABLE ACCESS BY INDEX ROWID BATCHED| EMP             |      1 |    38 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN                  | IF_EMP_HIREDATE |      1 |       |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1 / EMP@SEL$1
   2 - SEL$1 / EMP@SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter(("HIREDATE"<=TO_DATE(' 1980-12-18 00:00:00', 'syyyy-mm-dd hh24:mi:ss') AND
              "HIREDATE">=TO_DATE(' 1980-12-17 00:00:00', 'syyyy-mm-dd hh24:mi:ss')))
   2 - access("EMP"."SYS_NC00010$">=TRUNC(TO_DATE(' 1980-12-17 00:00:00', 'syyyy-mm-dd
              hh24:mi:ss')) AND "EMP"."SYS_NC00010$"<=TRUNC(TO_DATE(' 1980-12-18 00:00:00', 'syyyy-mm-dd
              hh24:mi:ss')))
--//一样有效.补充11g下执行计划:
SCOTT@book> @ &r/ver1
PORT_STRING                    VERSION        BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx            11.2.0.4.0     Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

SCOTT@book> select * from emp where hiredate between to_date('1980/12/16 09:00:00','yyyy-mm-dd hh24:mi:ss') and to_date('1980/12/18 10:00:00','yyyy-mm-dd hh24:mi:ss');
     EMPNO ENAME      JOB              MGR HIREDATE                   SAL       COMM     DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
      7369 SMITH      CLERK           7902 1980-12-17 00:00:00        800                    20

--//执行计划:

SQL_ID  f532a3utw8dfh, child number 0
-------------------------------------
select * from emp where hiredate between to_date('1980/12/16
09:00:00','yyyy-mm-dd hh24:mi:ss') and to_date('1980/12/18
10:00:00','yyyy-mm-dd hh24:mi:ss')

Plan hash value: 3187737602

------------------------------------------------------------------------------------------------
| Id  | Operation                   | Name            | E-Rows |E-Bytes| Cost (%CPU)| E-Time   |
------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                 |        |       |     2 (100)|          |
|*  1 |  TABLE ACCESS BY INDEX ROWID| EMP             |      1 |    46 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IF_EMP_HIREDATE |      1 |       |     1   (0)| 00:00:01 |
------------------------------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

   1 - SEL$1 / EMP@SEL$1
   2 - SEL$1 / EMP@SEL$1

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(("HIREDATE"<=TO_DATE(' 1980-12-18 10:00:00', 'syyyy-mm-dd hh24:mi:ss')
              AND "HIREDATE">=TO_DATE(' 1980-12-16 09:00:00', 'syyyy-mm-dd hh24:mi:ss')))
   2 - access("EMP"."SYS_NC00009$">=TRUNC(TO_DATE(' 1980-12-16 09:00:00', 'syyyy-mm-dd
              hh24:mi:ss')) AND "EMP"."SYS_NC00009$"<=TRUNC(TO_DATE(' 1980-12-18 10:00:00',
              'syyyy-mm-dd hh24:mi:ss')))


3.substr函数:
--//这个函数非常特殊,你建立的索引必须是substr(col,1,N)的格式,必须是从1开始截取,否则普通字段查询不会使用.这个很好理解.
--//看standard_hash函数后面的例子.

4.standard_hash函数:

--//缺点这个函数12c下才有.以前的测试,重复贴出,不想写了.
--//链接:http://blog.itpub.net/267265/viewspace-772856/
--//到目前为之,我仅仅知道这3个函数可以在普通字段查询时使用.
--//trunc,substr(Col,1,N)还合适范围查询.standard_hash(12c才有的函数)仅仅适合等值查询.

[20130916]12c Indexing Extended Data Types and index.txt

http://richardfoote.wordpress.com/2013/09/12/12c-indexing-extended-data-types-part-i-a-big-hurt/

--//参考以上链接,做一些测试:

1.测试环境:
SCOTT@test01p> @ver
BANNER                                                                               CON_ID
-------------------------------------------------------------------------------- ----------
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production              0

SCOTT@test01p> create table bowie (id number, text varchar2(32000));
Table created.

SCOTT@test01p> create index bowie_text_i on bowie(text);
create index bowie_text_i on bowie(text)
                             *
ERROR at line 1:
ORA-01450: maximum key length (6398) exceeded

--//超长字段无法在上建议索引。
--//12c提供standard_hash函数,可以实现其上建立函数索引。

2.插入一些数据,便于测试:
SCOTT@test01p> insert into bowie (id, text) values (1, lpad('a',1110,'a'));
1 row created.

SCOTT@test01p> commit ;
Commit complete.

SCOTT@test01p> select length(text) from bowie;
LENGTH(TEXT)
------------
        1110

SCOTT@test01p> insert into bowie (id, text) select 2, text||text||text||text||text||text||text||text||text||text from bowie;
1 row created.

SCOTT@test01p> commit ;
Commit complete.

SCOTT@test01p> select length(text) from bowie;
LENGTH(TEXT)
------------
        1110
       11100

SCOTT@test01p> insert into bowie (id, text) select rownum+2, to_char(rownum)||'BOWIE' from dual connect by level<=99998;
99998 rows created.

SCOTT@test01p> commit ;
Commit complete.

SCOTT@test01p> exec dbms_stats.gather_table_stats(ownname=>user, tabname=>'BOWIE', method_opt=>'FOR ALL COLUMNS SIZE 1');
PL/SQL procedure successfully completed.

3.建立函数索引:
SCOTT@test01p> create index bowie_hash_text_i on bowie(standard_hash(text));
Index created.

SCOTT@test01p> select index_name, num_rows, leaf_blocks from dba_indexes where index_name = 'BOWIE_HASH_TEXT_I';
INDEX_NAME           NUM_ROWS LEAF_BLOCKS
------------------ ---------- -----------
BOWIE_HASH_TEXT_I      100000         447

4.查询看看情况:
SCOTT@test01p> column text format a100
SCOTT@test01p> select * from bowie where text = '42BOWIE';
        ID TEXT
---------- ---------------------------------------------------
        44 42BOWIE

SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  3uz6tby2rv7bh, child number 1
-------------------------------------
select * from bowie where text = '42BOWIE'

Plan hash value: 1900956348

---------------------------------------------------------------------------------------
| Id  | Operation                           | Name              | E-Rows | Cost (%CPU)|
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |                   |        |     3 (100)|
|*  1 |  TABLE ACCESS BY INDEX ROWID BATCHED| BOWIE             |      1 |     3   (0)|
|*  2 |   INDEX RANGE SCAN                  | BOWIE_HASH_TEXT_I |      1 |     2   (0)|
---------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter((INTERNAL_FUNCTION("TEXT") AND
              INTERNAL_FUNCTION("TEXT")='42BOWIE'))
   2 - access("BOWIE"."SYS_NC00003$"=HEXTORAW('A2C98939EDB479BC3EB0CDC560DDCD15
              75D47F62'))
--可以发现可以使用这个函数索引。

5.但是这种情况存在一些限制,做like 或者between时,不能使用该函数索引:

SCOTT@test01p> select * from bowie where text like 'aaaaaaaaaaaaaaaaaaaaaa%';
...

SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------
SQL_ID  01fn3bq946un9, child number 0
-------------------------------------
select * from bowie where text like 'aaaaaaaaaaaaaaaaaaaaaa%'

Plan hash value: 1845943507

---------------------------------------------------------
| Id  | Operation         | Name  | E-Rows | Cost (%CPU)|
---------------------------------------------------------
|   0 | SELECT STATEMENT  |       |        |   208 (100)|
|*  1 |  TABLE ACCESS FULL| BOWIE |      1 |   208   (2)|
---------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter((INTERNAL_FUNCTION("TEXT") AND INTERNAL_FUNCTION("TEXT")
              LIKE 'aaaaaaaaaaaaaaaaaaaaaa%'))

SCOTT@test01p> select * from bowie where text between '4299BOWIE' and '42BOWIE';
        ID TEXT
---------- ---------------------------------------------------------------------
        44 42BOWIE
       431 429BOWIE
      4301 4299BOWIE

SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
SQL_ID  1uk9ud7fq8fdx, child number 0
-------------------------------------
select * from bowie where text between '4299BOWIE' and '42BOWIE'

Plan hash value: 1845943507

---------------------------------------------------------
| Id  | Operation         | Name  | E-Rows | Cost (%CPU)|
---------------------------------------------------------
|   0 | SELECT STATEMENT  |       |        |   208 (100)|
|*  1 |  TABLE ACCESS FULL| BOWIE |      2 |   208   (2)|
---------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter((INTERNAL_FUNCTION("TEXT") AND
              INTERNAL_FUNCTION("TEXT")<='42BOWIE' AND
              INTERNAL_FUNCTION("TEXT")>='4299BOWIE'))


SCOTT@test01p> select * from bowie where text > 'zzz';
no rows selected

SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
---------------------------------------------------------------
SQL_ID  39wfprrkz66td, child number 0
-------------------------------------
select * from bowie where text > 'zzz'

Plan hash value: 1845943507

---------------------------------------------------------
| Id  | Operation         | Name  | E-Rows | Cost (%CPU)|
---------------------------------------------------------
|   0 | SELECT STATEMENT  |       |        |   208 (100)|
|*  1 |  TABLE ACCESS FULL| BOWIE |      1 |   208   (2)|
---------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter((INTERNAL_FUNCTION("TEXT") AND
              INTERNAL_FUNCTION("TEXT")>'zzz'))


6.很明显,无法在该列上建议唯一约束。
SCOTT@test01p> alter table bowie add constraint bowie_text_unq unique (text);
alter table bowie add constraint bowie_text_unq unique (text)
*
ERROR at line 1:
ORA-01450: maximum key length (6398) exceeded

--//建立以上约束,需要在该列上建立索引,超长无法建立。同样可以变通的方法建立:

SCOTT@test01p> drop index bowie_hash_text_i;
Index dropped.

SCOTT@test01p> alter table bowie add (text_hash as (standard_hash(text)));
Table altered.

SCOTT@test01p> alter table bowie add constraint bowie_text_unq unique (text_hash);
Table altered.

SCOTT@test01p> insert into bowie (id, text) values (1000001, '42BOWIE');
insert into bowie (id, text) values (1000001, '42BOWIE')
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.BOWIE_TEXT_UNQ) violated

--再重复以上查询:
SCOTT@test01p> select * from bowie where text = '42BOWIE';
        ID TEXT     TEXT_HASH
---------- -------- ----------------------------------------
        44 42BOWIE  A2C98939EDB479BC3EB0CDC560DDCD1575D47F62

SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  3uz6tby2rv7bh, child number 1
-------------------------------------
select * from bowie where text = '42BOWIE'

Plan hash value: 2691947611

----------------------------------------------------------------------------
| Id  | Operation                   | Name           | E-Rows | Cost (%CPU)|
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |                |        |     2 (100)|
|*  1 |  TABLE ACCESS BY INDEX ROWID| BOWIE          |      1 |     2   (0)|
|*  2 |   INDEX UNIQUE SCAN         | BOWIE_TEXT_UNQ |      1 |     1   (0)|
----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter((INTERNAL_FUNCTION("TEXT") AND
              INTERNAL_FUNCTION("TEXT")='42BOWIE'))
   2 - access("BOWIE"."TEXT_HASH"=HEXTORAW('A2C98939EDB479BC3EB0CDC560DD
              CD1575D47F62'))

--但是如果做范围查询,结果如何应该同上是选择全表扫描。
SCOTT@test01p> select * from bowie where text between '429BOWIE' and '42BOWIE';
        ID TEXT      TEXT_HASH
---------- --------- ----------------------------------------
        44 42BOWIE   A2C98939EDB479BC3EB0CDC560DDCD1575D47F62
       431 429BOWIE  A7E2B59E1429DB4964225E7A98A19998BC3D2AFD

SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  143xd3cu22ja1, child number 0
-------------------------------------
select * from bowie where text between '429BOWIE' and '42BOWIE'

Plan hash value: 1845943507

---------------------------------------------------------
| Id  | Operation         | Name  | E-Rows | Cost (%CPU)|
---------------------------------------------------------
|   0 | SELECT STATEMENT  |       |        |   208 (100)|
|*  1 |  TABLE ACCESS FULL| BOWIE |      2 |   208   (2)|
---------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter((INTERNAL_FUNCTION("TEXT") AND
              INTERNAL_FUNCTION("TEXT")<='42BOWIE' AND
              INTERNAL_FUNCTION("TEXT")>='429BOWIE'))

7.如果做范围查询如何显示呢?期待作者的第2部分new extended columns in Part II.
自己也想一下,实际上作者的例子,text组成前面数字+BOWIE。前面5位具有很好的选择性。通过函数substr建立函数应该也可以,
自己测试看看。

SCOTT@test01p> create index i_bowie_text_substr_1_5 on bowie (substr(text,1,5));
Index created.

SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  143xd3cu22ja1, child number 0
-------------------------------------
select * from bowie where text between '429BOWIE' and '42BOWIE'

Plan hash value: 1199225668

---------------------------------------------------------------------------------------------
| Id  | Operation                           | Name                    | E-Rows | Cost (%CPU)|
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |                         |        |    92 (100)|
|*  1 |  TABLE ACCESS BY INDEX ROWID BATCHED| BOWIE                   |      2 |    92   (0)|
|*  2 |   INDEX RANGE SCAN                  | I_BOWIE_TEXT_SUBSTR_1_5 |    450 |     3   (0)|
---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter((INTERNAL_FUNCTION("TEXT") AND INTERNAL_FUNCTION("TEXT")<='42BOWIE'
              AND INTERNAL_FUNCTION("TEXT")>='429BOWIE'))
   2 - access("BOWIE"."SYS_NC00004$">='429BO' AND "BOWIE"."SYS_NC00004$"<='42BOW')


--可以发现使用我建立的索引,看看使用大于的情况呢?
SCOTT@test01p> select * from bowie where text > 'zzz';
no rows selected

SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  39wfprrkz66td, child number 0
-------------------------------------
select * from bowie where text > 'zzz'
Plan hash value: 1199225668
---------------------------------------------------------------------------------------------
| Id  | Operation                           | Name                    | E-Rows | Cost (%CPU)|
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |                         |        |   181 (100)|
|*  1 |  TABLE ACCESS BY INDEX ROWID BATCHED| BOWIE                   |      1 |   181   (0)|
|*  2 |   INDEX RANGE SCAN                  | I_BOWIE_TEXT_SUBSTR_1_5 |    900 |     4   (0)|
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter((INTERNAL_FUNCTION("TEXT") AND INTERNAL_FUNCTION("TEXT")>'zzz'))
   2 - access("BOWIE"."SYS_NC00004$">='zzz')

--//依旧可以使用我建立的函数索引,但是使用like情况如何呢?

SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  1mq1xczjrz3uw, child number 0
-------------------------------------
select * from bowie where text like 'aaaaaaaaaaaaaaaaaaaaaa%'

Plan hash value: 1845943507

---------------------------------------------------------
| Id  | Operation         | Name  | E-Rows | Cost (%CPU)|
---------------------------------------------------------
|   0 | SELECT STATEMENT  |       |        |   208 (100)|
|*  1 |  TABLE ACCESS FULL| BOWIE |      1 |   208   (2)|
---------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter((INTERNAL_FUNCTION("TEXT") AND INTERNAL_FUNCTION("TEXT")
              LIKE 'aaaaaaaaaaaaaaaaaaaaaa%'))

--//like 无效,不知道作者还有什么好方法,期待作者的第2部分,也许有更好的例子。
--//改写为范围查询也许是一个替换like的方法,但是不适合'%aaaa%'的情况。

SCOTT@test01p>  select * from bowie where text between 'aaaaaaaaaaaaaaaaaaaaaa' and 'aaaaaaaaaaaaaaaaaaaaaa'||chr(255);
SCOTT@test01p> @dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  bkunhv8x64k0a, child number 1
-------------------------------------
 select * from bowie where text between 'aaaaaaaaaaaaaaaaaaaaaa' and
'aaaaaaaaaaaaaaaaaaaaaa'||chr(255)

Plan hash value: 1199225668

---------------------------------------------------------------------------------------------
| Id  | Operation                           | Name                    | E-Rows | Cost (%CPU)|
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |                         |        |     2 (100)|
|*  1 |  TABLE ACCESS BY INDEX ROWID BATCHED| BOWIE                   |      1 |     2   (0)|
|*  2 |   INDEX RANGE SCAN                  | I_BOWIE_TEXT_SUBSTR_1_5 |      2 |     1   (0)|
---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter((INTERNAL_FUNCTION("TEXT") AND
              INTERNAL_FUNCTION("TEXT")>='aaaaaaaaaaaaaaaaaaaaaa' AND
              INTERNAL_FUNCTION("TEXT")<='aaaaaaaaaaaaaaaaaaaaaa'))
   2 - access("BOWIE"."SYS_NC00004$"='aaaaa')

--//总结:
--//BTW:如果字符字段很长,使用substr函数取前面选择性很强的几位,建立函数索引,有时候不失为一个方法,这样可以减少索引大小,
--//这种方法在10G,11G使用同样有效。
--//12C extended columns中提供的standard_hash函数,作为等值查询,不失为一个很好的选择。