识别低效率的SQL语句

1.返回行与逻辑读的比率

CREATE TABLE t as select * from dba_objects;

--CREATE INDEX idx ON t (object_id);



---例1

alter session set statistics_level=all;



set linesize 1000

set pagesize 2000

select * from t where object_id=6;



SELECT * FROM table(dbms_xplan.display_cursor(NULL,NULL,'allstats last'));



PLAN_TABLE_OUTPUT

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

SQL_ID  8cxbzma1az713, child number 0

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

select * from t where object_id=6



Plan hash value: 1601196873

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

| Id  | Operation         | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |

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

|   0 | SELECT STATEMENT  |      |      1 |        |      1 |00:00:00.07 |    1048 |    774 |

|*  1 |  TABLE ACCESS FULL| T    |      1 |     12 |      1 |00:00:00.07 |    1048 |    774 |

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

Predicate Information (identified by operation id):

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

   1 - filter("OBJECT_ID"=6)

Note

-----

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

上面的语句只返回了1行数据却产生了1048个逻辑读。

执行计划显示的是全表扫描，创建索引

CREATE INDEX idx ON t (object_id);

执行计划如下：

select * from t where object_id=6



Plan hash value: 2770274160



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

| Id  | Operation                   | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

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

|   0 | SELECT STATEMENT            |      |      1 |        |      1 |00:00:00.01 |       4 |

|   1 |  TABLE ACCESS BY INDEX ROWID| T    |      1 |      1 |      1 |00:00:00.01 |       4 |

|*  2 |   INDEX RANGE SCAN          | IDX  |      1 |      1 |      1 |00:00:00.01 |       3 |

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

逻辑读为4。

 

2.执行计划中的评估是否准确。

查看e-rows 预估数量，a-rows 实际返回的数量。如果相差过大则说明需要收集表的统计信息。

SELECT * FROM table(dbms_xplan.display_cursor(NULL,NULL,'allstats last'));

 

3.类型转换需要关注。

举例如下：

create table t_col_type(id varchar2(20),col2 varchar2(20),col3 varchar2(20));

insert into t_col_type select rownum,'abc','efg' from dual connect by level<=10000;

commit;
create index idx_id on t_col_type(id);

注意ID的数据类型为VARCHAR(20)

select * from t_col_type where id=6;

执行计划

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

Plan hash value: 3191204463

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

| Id  | Operation         | Name       | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT  |            |     1 |    36 |     9   (0)| 00:00:01 |

|*  1 |  TABLE ACCESS FULL| T_COL_TYPE |     1 |    36 |     9   (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

 1 - filter(TO_NUMBER("ID")=6)

注意这里没有使用到索引。

执行

select * from t_col_type where id='6';

执行计划

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

Plan hash value: 3998173245

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

| Id  | Operation                   | Name       | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT            |            |     1 |    36 |     2   (0)| 00:00:01 |

|   1 |  TABLE ACCESS BY INDEX ROWID| T_COL_TYPE |     1 |    36 |     2   (0)| 00:00:01 |

|*  2 |   INDEX RANGE SCAN          | IDX_ID     |     1 |       |     1   (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

  2 - access("ID"='6')

使用到了索引。

 

4.小心递归调用。

6种获取执行计划的方法中，只有 autotrace 的方式可以看出递归调用的次数（recursive calls）

注意SQL中尽量不要使用函数的使用例如：

drop table people purge;

create table people (first_name varchar2(200),last_name varchar2(200),sex_id number);



create table sex (name varchar2(20), sex_id number);

insert into people (first_name,last_name,sex_id) select object_name,object_type,1 from dba_objects;

insert into sex (name,sex_id) values ('男',1);

insert into sex (name,sex_id) values ('女',2);

insert into sex (name,sex_id) values ('不详',3);

commit;





create or replace function get_sex_name(p_id sex.sex_id%type) return sex.name%type is

v_name sex.name%type;

begin

select name

into v_name

from sex

where sex_id=p_id;

return v_name;

end;

/

执行：

set linesize 1000

set pagesize 2000



set autotrace traceonly



--例1：



select sex_id,

first_name||' '||last_name full_name,

get_sex_name(sex_id) gender

from people;

执行计划如下：

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

Plan hash value: 2528372185

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

| Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |

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

|   0 | SELECT STATEMENT  |        | 80635 |    16M|   137   (1)| 00:00:02 |

|   1 |  TABLE ACCESS FULL| PEOPLE | 80635 |    16M|   137   (1)| 00:00:02 |

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

Note

-----

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



统计信息

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

    73121 recursive calls 0  db block gets

     517142  consistent gets

          0  physical reads

          0  redo size

    3382143  bytes sent via SQL*Net to client

      54029  bytes received via SQL*Net from client

       4876  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

      73121  rows processed

产生了73121此递归调用。

消除办法，直接使用关联查询。

 

5.表的访问次数。

 6种获取执行计划的方法中，只有 statisitcs_level=all 的方式可以看出表访问次数（STARTS)，这个很重要！

 执行：

SELECT /*+ gather_plan_statistics */ count(t2.col2)

FROM t1 ,t2 WHERE t1.id=t2.id and t1.col1 = 666;

SELECT * FROM table(dbms_xplan.display_cursor(NULL,NULL,'allstats last'));

PLAN_TABLE_OUTPUT

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

SQL_ID  g048suxnxkxyr, child number 0

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

SELECT /*+ gather_plan_statistics */ count(t2.col2) FROM t1 ,t2 WHERE

t1.id=t2.id and t1.col1 = 666



Plan hash value: 3711554156

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

| Id  | Operation                      | Name    | Starts | E-Rows | A-Rows |   A-Time   | Buffers |

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

|   0 | SELECT STATEMENT               |         |      1 |        |      1 |00:00:00.30 |   94651 |

|   1 |  SORT AGGREGATE                |         |      1 |      1 |      1 |00:00:00.30 |   94651 |

|   2 |   NESTED LOOPS                 |         |      1 |        |  75808 |00:00:00.31 |   94651 |

|   3 |    NESTED LOOPS                |         |      1 |     32 |  75808 |00:00:00.19 |   18843 |

|   4 |     TABLE ACCESS BY INDEX ROWID| T1      |      1 |     32 |  80016 |00:00:00.08 |    1771 |

|*  5 |      INDEX RANGE SCAN          | T1_COL1 |      1 |     32 |  80016 |00:00:00.03 |     169 |

|*  6 |     INDEX UNIQUE SCAN          | T2_PK   |  80016 |      1 |  75808 |00:00:00.08 |   17072 |

|   7 |    TABLE ACCESS BY INDEX ROWID | T2      |  75808 |      1 |  75808 |00:00:00.08 |   75808 |

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

Predicate Information (identified by operation id):

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

   5 - access("T1"."COL1"=666)

   6 - access("T1"."ID"="T2"."ID")

这里表的访问次数过大，应该走hash或排序合并连接，原因是表的收集信息不准确。

NL连接表的访问次数不会这么大。

 

6.注意表的真实访问行数。

数据准备：

drop table t1 cascade constraints;

create table t1 as select * from dba_objects;

drop table t2 cascade constraints;

create table t2 (id1,id2) as 

select rownum ,rownum+100 from dual connect  by level <=1000;



alter session set statistics_level=all;

set linesize 1000

set pagesize 2000

优化前执行如下：

select *

  from (select t1.*, rownum as rn from t1, t2 where t1.object_id = t2.id1) a

 where a.rn >= 1

   and a.rn <= 10;

SELECT * FROM table(dbms_xplan.display_cursor(NULL,NULL,'allstats last'));



SQL_ID  ayzfn8k0j3sms, child number 0

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

select *   from (select t1.*, rownum as rn from t1, t2 where

t1.object_id = t2.id1) a  where a.rn >= 1    and a.rn <= 10



Plan hash value: 3062220019

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

| Id  | Operation            | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |  OMem |  1Mem | Used-Mem |

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

|   0 | SELECT STATEMENT     |      |      1 |        |     10 |00:00:00.11 |    1052 |    749 |       |       |          |

|*  1 |  VIEW                |      |      1 |   1008 |     10 |00:00:00.11 |    1052 |    749 |       |       |          |

|   2 |   COUNT              |      |      1 |        |    943 |00:00:00.11 |    1052 |    749 |       |       |          |

|*  3 |    HASH JOIN         |      |      1 |   1008 |    943 |00:00:00.11 |    1052 |    749 |  1036K|  1036K| 1197K (0)|

|   4 |     TABLE ACCESS FULL| T2   |      1 |   1000 |   1000 |00:00:00.01 |       4 |      0 |       |       |          |

|   5 |     TABLE ACCESS FULL| T1   |      1 |  70183 |  73156 |00:00:00.08 |    1048 |    749 |       |       |          |

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

Predicate Information (identified by operation id):

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

   1 - filter(("A"."RN"<=10 AND "A"."RN">=1))

   3 - access("T1"."OBJECT_ID"="T2"."ID1")

Note

-----

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

这个查询总共返回10记录，但是内部查询返回了73156  条记录。

 

优化后：

select *

  from (select t1.*, rownum as rn from t1, t2 where t1.object_id = t2.id1 and rownum<=10) a

 where a.rn >= 1; 

 

SELECT * FROM table(dbms_xplan.display_cursor(NULL,NULL,'allstats last'));

PLAN_TABLE_OUTPUT

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

SQL_ID  7wzvqay91x14y, child number 0

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

select *   from (select t1.*, rownum as rn from t1, t2 where

t1.object_id = t2.id1 and rownum<=10) a  where a.rn >= 1



Plan hash value: 1802812661

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

| Id  | Operation            | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |

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

|   0 | SELECT STATEMENT     |      |      1 |        |     10 |00:00:00.01 |       9 |       |       |          |

|*  1 |  VIEW                |      |      1 |     10 |     10 |00:00:00.01 |       9 |       |       |          |

|*  2 |   COUNT STOPKEY      |      |      1 |        |     10 |00:00:00.01 |       9 |       |       |          |

|*  3 |    HASH JOIN         |      |      1 |   1008 |     10 |00:00:00.01 |       9 |  1036K|  1036K| 1210K (0)|

|   4 |     TABLE ACCESS FULL| T2   |      1 |   1000 |   1000 |00:00:00.01 |       4 |       |       |          |

|   5 |     TABLE ACCESS FULL| T1   |      1 |  70183 |     10 |00:00:00.01 |       5 |       |       |          |

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

Predicate Information (identified by operation id):

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

   1 - filter("A"."RN">=1)

   2 - filter(ROWNUM<=10)

   3 - access("T1"."OBJECT_ID"="T2"."ID1")

Note

-----

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

T1表只返回十条数据。

这种修改 可以优化分页数据。

第一页

select *

from (select t1.*, rownum as rn from t1, t2 where t1.object_id = t2.id1 and rownum<=10) a

where a.rn >= 1;



第二页



select *

from (select t1.*, rownum as rn from t1, t2 where t1.object_id = t2.id1 and rownum<=20) a

where a.rn >= 10;



第三页



select *

from (select t1.*, rownum as rn from t1, t2 where t1.object_id = t2.id1 and rownum<=30) a

where a.rn >= 20;

这样，可以提高前几页的分页效率。

 

7.使用索引消除排序。

比如需要根据object_id 进行排序，那么可以使用索引消除排序操作，因为索引本身有序。

create index idx_object_id on t(object_id);
set autotrace traceonly
select * from t where object_id>2 order by object_id;