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;