1.建立测试环境

SQL> select * from v$version ;

BANNER

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

Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production

PL/SQL Release 11.2.0.1.0 - Production

CORE    11.2.0.1.0      Production

TNS for Linux: Version 11.2.0.1.0 - Production

NLSRTL Version 11.2.0.1.0 - Production

create table t as select rownum id ,cast(dbms_random.string('x',16) as varchar2(16)) name,lpad('x',100) text from dual connect by level

SQL> create index i_t_name on t(name);

Index created.

SQL> explain plan for select * from t where name=:x;

Explained.

SQL> select * from table(dbms_xplan.display(NULL,NULL,'ALLSTATS LAST PEEKED_BINDS cost'));

PLAN_TABLE_OUTPUT

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

Plan hash value: 1766912905

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

| Id  | Operation                   | Name     | E-Rows | Cost (%CPU)|

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

|   0 | SELECT STATEMENT            |          |    115 |    41   (0)|

|   1 |  TABLE ACCESS BY INDEX ROWID| T        |    115 |    41   (0)|

|*  2 |   INDEX RANGE SCAN          | I_T_NAME |     46 |     1   (0)|

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

Predicate Information (identified by operation id):

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

   2 - access("NAME"=:X)

Note

-----

   - dynamic sampling used for this statement (level=2)

   - Warning: basic plan statistics not available. These are only collected when:

       * hint 'gather_plan_statistics' is used for the statement or

       * parameter 'statistics_level' is set to 'ALL', at session or system level

21 rows selected.

--建立函数索引

SQL> create index if_t_name on t(substr(name,1,5));

Index created.

SQL> explain plan for select * from t where name=:x;

Explained.

SQL> select * from table(dbms_xplan.display(NULL,NULL,'ALLSTATS LAST PEEKED_BINDS cost'));

PLAN_TABLE_OUTPUT

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

Plan hash value: 1766912905

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

| Id  | Operation                   | Name     | E-Rows | Cost (%CPU)|

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

|   0 | SELECT STATEMENT            |          |      1 |     2   (0)|

|   1 |  TABLE ACCESS BY INDEX ROWID| T        |      1 |     2   (0)|

|*  2 |   INDEX RANGE SCAN          | I_T_NAME |      1 |     1   (0)|

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

Predicate Information (identified by operation id):

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

   2 - access("NAME"=:X)

       filter(SUBSTR("NAME",1,5)=SUBSTR(:X,1,5))

Note

-----

   - dynamic sampling used for this statement (level=2)

   - Warning: basic plan statistics not available. These are only collected when:

       * hint 'gather_plan_statistics' is used for the statement or

       * parameter 'statistics_level' is set to 'ALL', at session or system level

22 rows selected.

--如果按照执行计划的内容,实际上sql语句被转化为如下:
select * from t where name=:x and SUBSTR("NAME",1,5)=SUBSTR(:X,1,5);
--实际的执行计划好像没用filter(SUBSTR("NAME",1,5)=SUBSTR(:X,1,5)).explain 看带绑定的sql语句至少说明这样的计划是可行的.
--不使用环境变量看看.

SQL> select id,name from t where rownum=1;

        ID NAME

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

         1 23C1W6VWH59ORH12

SQL> select * from t where name='23C1W6VWH59ORH12';

        ID NAME             TEXT

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

         1 23C1W6VWH59ORH12        x

SQL> @dpc

PLAN_TABLE_OUTPUT

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

SQL_ID  cybmv2qjraur2, child number 0

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

select * from t where name='23C1W6VWH59ORH12'

Plan hash value: 1766912905

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

| Id  | Operation                   | Name     | E-Rows | Cost (%CPU)|

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

|   0 | SELECT STATEMENT            |          |        |     2 (100)|

|   1 |  TABLE ACCESS BY INDEX ROWID| T        |      3 |     2   (0)|

|*  2 |   INDEX RANGE SCAN          | I_T_NAME |      1 |     1   (0)|

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

Predicate Information (identified by operation id):

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

   2 - access("NAME"='23C1W6VWH59ORH12')

Note

-----

   - dynamic sampling used for this statement (level=2)

   - Warning: basic plan statistics not available. These are only collected when:

       * hint 'gather_plan_statistics' is used for the statement or

       * parameter 'statistics_level' is set to 'ALL', at session or system level

26 rows selected.

3.分析表看看:

SQL> exec dbms_stats.gather_table_stats(user,'t');

PL/SQL procedure successfully completed.

SQL> explain plan for select * from t where name=:x;

Explained.

SQL> select * from table(dbms_xplan.display(NULL,NULL,'ALLSTATS LAST PEEKED_BINDS cost'));

PLAN_TABLE_OUTPUT

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

Plan hash value: 3367455390

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

| Id  | Operation                   | Name      | E-Rows | Cost (%CPU)|

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

|   0 | SELECT STATEMENT            |           |      1 |     2   (0)|

|*  1 |  TABLE ACCESS BY INDEX ROWID| T         |      1 |     2   (0)|

|*  2 |   INDEX RANGE SCAN          | IF_T_NAME |      1 |     1   (0)|

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

Predicate Information (identified by operation id):

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

   1 - filter("NAME"=:X)

   2 - access(SUBSTR("NAME",1,5)=SUBSTR(:X,1,5))

Note

-----

   - Warning: basic plan statistics not available. These are only collected when:

       * hint 'gather_plan_statistics' is used for the statement or

       * parameter 'statistics_level' is set to 'ALL', at session or system level

21 rows selected.

--这个对于我是颠覆性的,查询name=:x条件,而可以使用IF_T_NAME的索引.

4.使用提示看看:

SQL> variable x varchar2(30);

SQL> exec :x := '23C1W6VWH59ORH12';

PL/SQL procedure successfully completed.

SQL> select /*+ index (t if_t_name ) */ * from t where name=:x;

        ID NAME             TEXT

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

         1 23C1W6VWH59ORH12       x  

SQL> @dpc

PLAN_TABLE_OUTPUT

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

SQL_ID  337p0r8k0cmkb, child number 0

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

select /*+ index (t if_t_name ) */ * from t where name=:x

Plan hash value: 3367455390

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

| Id  | Operation                   | Name      | E-Rows | Cost (%CPU)|

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

|   0 | SELECT STATEMENT            |           |        |     2 (100)|

|*  1 |  TABLE ACCESS BY INDEX ROWID| T         |      1 |     2   (0)|

|*  2 |   INDEX RANGE SCAN          | IF_T_NAME |      1 |     1   (0)|

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

Peeked Binds (identified by position):

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

   1 - (VARCHAR2(30), CSID=852): '23C1W6VWH59ORH12'

Predicate Information (identified by operation id):

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

   1 - filter("NAME"=:X)

   2 - access("T"."SYS_NC00004$"=SUBSTR(:X,1,5))

Note

-----

   - Warning: basic plan statistics not available. These are only collected when:

       * hint 'gather_plan_statistics' is used for the statement or

       * parameter 'statistics_level' is set to 'ALL', at session or system level

31 rows selected.

--可以发现使用提示在查询name=:x 这样的条件时,可以IF_T_NAME这样的函数索引.

--删除索引i_t_name,再看看情况如何呢?

SQL> drop index i_t_name;

Index dropped.

SQL> select  * from t where name=:x;

        ID NAME             TEXT

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

         1 23C1W6VWH59ORH12                                                       

SQL> @dpc

PLAN_TABLE_OUTPUT

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

SQL_ID  69yvh72cyvwsx, child number 0

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

select  * from t where name=:x

Plan hash value: 3367455390

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

| Id  | Operation                   | Name      | E-Rows | Cost (%CPU)|

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

|   0 | SELECT STATEMENT            |           |        |     2 (100)|

|*  1 |  TABLE ACCESS BY INDEX ROWID| T         |      1 |     2   (0)|

|*  2 |   INDEX RANGE SCAN          | IF_T_NAME |      1 |     1   (0)|

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

Peeked Binds (identified by position):

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

   1 - (VARCHAR2(30), CSID=852): '23C1W6VWH59ORH12'

Predicate Information (identified by operation id):

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

   1 - filter("NAME"=:X)

   2 - access("T"."SYS_NC00004$"=SUBSTR(:X,1,5))

Note

-----

   - Warning: basic plan statistics not available. These are only collected when:

       * hint 'gather_plan_statistics' is used for the statement or

       * parameter 'statistics_level' is set to 'ALL', at session or system level

31 rows selected.

--这样可以得出结论,查询name=:x这样的条件,可以通过建立substr(name,1,5)这样的函数索引来访问.
--这样可以想象一种情况,比如name的长度很长,如果前面的几个字符选择性很好,可以通过使用substr函数建立这样的索引来访问表.好处就是这样索引可以建立的很小.