[20160706]like % 绑定变量.txt
--最近一直在优化一个项目，程序中存在大量的like模糊查询，例子：
/* Formatted on 2016/7/6 11:10:55 (QP5 v5.252.13127.32867) */
SELECT PATIENT_ID
,NAME
,RIS_NO
,SEX
,EXAM_ITEM
,DATE_OF_BIRTH
,LASTSAVEUSER
,EXAM_PARA
,PATIENT_LOCAL_ID
,MODALITY
,EXAM_CLASS
,EXAM_SUB_CLASS
,DEVICE
,RESULT_STATUS
,EXAM_DATE_TIME
,STUDY_UID
,LASTSAVETIME
,CHECKUSER
,NORMALNAME
,CHECKSTATE
,OUT_URL
,CHECKUSERID
,CREATEUSER
,CHECKTIME
,PRINT_STATUS
,IS_ABNORMAL
FROM REPORTQUERY
WHERE patient_id LIKE :V001
AND (LASTSAVETIME BETWEEN :V002 AND :V003)
AND (checkstate = :V004)
AND (modality = :V005)
AND ROWNUM < 1001
SQL_ID CHILD_NUMBER WAS NAME POSITION MAX_LENGTH LAST_CAPTURED DATATYPE_STRING VALUE_STRING
------------- ------------ --- -------------------- ---------- ---------- ------------------- --------------- --------------------
222m95hh0kvgh 1 YES :V001 1 128 2016-07-02 10:53:34 VARCHAR2(128) %TJ1431070%
YES :V002 2 32 2016-07-02 10:53:34 VARCHAR2(32) 2016-07-02 00:00:00
YES :V003 3 32 2016-07-02 10:53:34 VARCHAR2(32) 2016-07-02 23:59:59
YES :V004 4 2000 2016-07-02 10:53:34 VARCHAR2(2000) 40
YES :V005 5 128 2016-07-02 10:53:34 VARCHAR2(128) US
--//REPORTQUERY是视图，本来选择patient_id是很快的查询，由于前面的%,导致执行计划选择LASTSAVETIME，而这个范围是1天(有时候查询1个月)，导致
--//大量的逻辑读。我自己做一些测试想看看使用绑定变量的情况。
1.环境：
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
2.建立测试例子：
create table tx (id number,v varchar2(20),nv nvarchar2(20),pad varchar2(200));
insert into tx select rownum,lpad(rownum,6,'0'),lpad(rownum,6,'0'),lpad('x',200,'x') from dual connect by level<=1e5;
commit ;
create index i_tx_v on tx(v) PCTFREE 50;
create index i_tx_nv on tx(nv) PCTFREE 50;
execute sys.dbms_stats.gather_table_stats ( OwnName => user,TabName => 'tx',Estimate_Percent => NULL,Method_Opt => 'FOR ALL COLUMNS SIZE 1 ',Cascade => True ,No_Invalidate => false);
3.测试：
SCOTT@book> column pad noprint
SCOTT@book> alter session set statistics_level=all;
Session altered.
SCOTT@book> select * from tx where v like '%000042%';
ID V NV
---------- -------------------- ----------------------------------------
42 000042 000042
SCOTT@book> @ &r/dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID c9z5cg37cu8yz, child number 0
-------------------------------------
select * from tx where v like '%000042%'
Plan hash value: 40191160
--------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
--------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 888 (100)| | 1 |00:00:00.06 | 3275 |
|* 1 | TABLE ACCESS FULL| TX | 1 | 5000 | 1103K| 888 (1)| 00:00:11 | 1 |00:00:00.06 | 3275 |
--------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1 / TX@SEL$1
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(("V" LIKE '%000042%' AND "V" IS NOT NULL))
--如果查询select * from tx where v like '000042%';是可以使用索引的。
4.如果使用绑定变量呢?
SCOTT@book> variable x varchar2(20);
SCOTT@book> exec :x := '%000042%';
PL/SQL procedure successfully completed.
SCOTT@book> select * from tx where v like :x;
ID V NV
---------- -------------------- ----------------------------------------
42 000042 000042
SCOTT@book> @ &r/dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 87rcszfzcz5gu, child number 0
-------------------------------------
select * from tx where v like :x
Plan hash value: 3964412060
-----------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | Reads |
-----------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 187 (100)| | 1 |00:00:00.07 | 464 | 461 |
| 1 | TABLE ACCESS BY INDEX ROWID| TX | 1 | 5000 | 1103K| 187 (0)| 00:00:03 | 1 |00:00:00.07 | 464 | 461 |
|* 2 | INDEX RANGE SCAN | I_TX_V | 1 | 5000 | | 25 (0)| 00:00:01 | 1 |00:00:00.07 | 463 | 461 |
-----------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1 / TX@SEL$1
2 - SEL$1 / TX@SEL$1
Peeked Binds (identified by position):
--------------------------------------
1 - (VARCHAR2(30), CSID=852): '%000042%'
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("V" LIKE :X)
filter("V" LIKE :X)
--可以发现绑定变量是使用索引的。有点奇怪的是估计返回相同的行相同，而带入参数选择索引，估计索引群集因子很小。
--把索引建立大一些看看。
SCOTT@book> alter index i_tx_v rebuild pctfree 90;
Index altered.
SCOTT@book> execute sys.dbms_stats.gather_table_stats ( OwnName => user,TabName => 'tx',Estimate_Percent => NULL,Method_Opt => 'FOR ALL COLUMNS SIZE 1 ',Cascade => True ,No_Invalidate => false);
PL/SQL procedure successfully completed.
SCOTT@book> select * from tx where v like :x;
ID V NV
---------- -------------------- ----------------------------------------
42 000042 000042
SCOTT@book> @ &r/dpc '' ''
PLAN_TABLE_OUTPUT
--------------------------------------
SQL_ID 87rcszfzcz5gu, child number 0
-------------------------------------
select * from tx where v like :x
Plan hash value: 3964412060
--------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
--------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 307 (100)| | 1 |00:00:00.07 | 2862 |
| 1 | TABLE ACCESS BY INDEX ROWID| TX | 1 | 5000 | 1103K| 307 (0)| 00:00:04 | 1 |00:00:00.07 | 2862 |
|* 2 | INDEX RANGE SCAN | I_TX_V | 1 | 5000 | | 145 (0)| 00:00:02 | 1 |00:00:00.07 | 2861 |
--------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1 / TX@SEL$1
2 - SEL$1 / TX@SEL$1
Peeked Binds (identified by position):
--------------------------------------
1 - (VARCHAR2(30), CSID=852): '%000042%'
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("V" LIKE :X)
filter("V" LIKE :X)
--//这样并没有改变群集因子,cost增加不大。
--//oracle 估计返回5000行，占5000/100000=5%.oracle也是选择索引范围扫描。
--//这种情况是选择索引好还是全表扫描还呢？主要看返回的行数量。
--//oracle很奇怪，前面的直接打入文字变量，估计返回也是5000，但是执行计划选择的是全表扫描。
SCOTT@book> exec :x := '%00%';
PL/SQL procedure successfully completed.
Plan hash value: 3964412060
--------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
--------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 307 (100)| | 12520 |00:00:00.08 | 4242 |
| 1 | TABLE ACCESS BY INDEX ROWID| TX | 1 | 5000 | 1103K| 307 (0)| 00:00:04 | 12520 |00:00:00.08 | 4242 |
|* 2 | INDEX RANGE SCAN | I_TX_V | 1 | 5000 | | 145 (0)| 00:00:02 | 12520 |00:00:00.07 | 2923 |
--------------------------------------------------------------------------------------------------------------------------------
--执行计划不变。重新建立表ty。大乱顺序，提高索引的群集因子。
SCOTT@book> create table ty as select * from tx order by DBMS_RANDOM.VALUE;
Table created.
create index i_ty_v on ty(v) PCTFREE 10;
create index i_ty_nv on ty(nv) PCTFREE 10;
--分析表，忽略。
SCOTT@book> exec :x := '%00%';
PL/SQL procedure successfully completed.
SCOTT@book> @ &r/dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 50t5atjp1pjh9, child number 0
-------------------------------------
select * from ty where v like :x
Plan hash value: 1260447134
-----------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | Reads |
-----------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 893 (100)| | 12520 |00:00:00.07 | 3284 | 3226 |
|* 1 | TABLE ACCESS FULL| TY | 1 | 5000 | 1103K| 893 (1)| 00:00:11 | 12520 |00:00:00.07 | 3284 | 3226 |
-----------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1 / TY@SEL$1
Peeked Binds (identified by position):
--------------------------------------
1 - (VARCHAR2(30), CSID=852): '%00%'
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("V" LIKE :X)
--可以看到群集因子对执行计划影响很大。实际上这种执行计划如果返回行很少，使用索引更佳。
--//我个人的建议最好少用模糊查询，开发往往说这个是用户要求，实际上如果最后很慢了，用户往往抱怨不断。我自己很久以前就遇到类似的情况。
--//我还建议开发可以做一个按钮，缺省做精确查询，用户要模糊查询必须选中它。如果模糊很慢，以后用户自然会少用，可惜大部分开发不愿意做.....
--//当然现在有许多方法来稳定执行计划。sql profile ,spm都可以实现。但是实际上这种是以扫描整个索引为代价，而且是单块读。
--//我以前的做法就是人为修改统计信息，lock统计，增加表的块数量，导致执行计划选择索引，因为大多数情况返回的行很少，很多查询基本没有意义。
--//比如你查询 name like '%李%'，这样返回很多行，用户不大可能在里面查找需要的信息。
SCOTT@book> EXECUTE SYS.DBMS_STATS.set_table_stats (OWNNAME=>user, TABNAME=>'ty', numblks=> 8888888);
PL/SQL procedure successfully completed.
SCOTT@book> select * from ty where v like :x;
ID V NV
---------- -------------------- ----------------------------------------
42 000042 000042
SCOTT@book> @ &r/dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 50t5atjp1pjh9, child number 1
-------------------------------------
select * from ty where v like :x
Plan hash value: 1299836486
-----------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | Reads |
-----------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 5014 (100)| | 1 |00:00:00.06 | 255 | 1 |
| 1 | TABLE ACCESS BY INDEX ROWID| TY | 1 | 5000 | 1103K| 5014 (1)| 00:01:01 | 1 |00:00:00.06 | 255 | 1 |
|* 2 | INDEX RANGE SCAN | I_TY_V | 1 | 5000 | | 14 (0)| 00:00:01 | 1 |00:00:00.06 | 254 | 0 |
-----------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1 / TY@SEL$1
2 - SEL$1 / TY@SEL$1
Peeked Binds (identified by position):
--------------------------------------
1 - (VARCHAR2(30), CSID=852): '%000042%'
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("V" LIKE :X)
filter("V" LIKE :X)
--总之，开发写代码不要太随意了，认真思考可能产生的结果。看问题看得长远一些。
--当使用模糊like时，里面的查询条件一定要注意。
