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='';
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  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;