论Optimizer的工作模式ALL_ROWS&FIRST_ROWS
优化器是oracle用于分析SQL语句和执行对象的一个核心工具，8i之前oracle使用的是RBO（基于规则优化器），9i 10g 11g 已经升级为CBO（基于成本优化器），例如要根据操作系统cpu资源；内存资源；磁盘I/O资源；实例参数；表对象；索引对象；列对象等内容综合计算出不同的成本，对比哪种成本最优从而选择出最适合的方案。
CBO Optimizer 有2种工作模式：
1. all_rows：这种工作模式要求一次性处理完全部的数据返回给用户，场合：报表系统，金融系统
2. first_rows(n)：这种工作模式要求把前n条记录马上处理完优先返回给用户，场合：搜索，论坛，电商推荐，网上购物
下面就用实例来对比一下all_rows和first_rows(n)性能差异
LEO1@LEO1>create table leo1 as select * from dba_objects; 创建leo1表
Table created.
143916 rowscreated.
LEO1@LEO1>insert /*+ parellel */ into leo1 select * from leo1;
287832 rowscreated.
LEO1@LEO1>insert /*+ parellel */ into leo1 select * from leo1;
575664 rowscreated.
LEO1@LEO1>insert /*+ parellel */ into leo1 select * from leo1;
1151328 rowscreated.
LEO1@LEO1>commit;
Commit complete.
LEO1@LEO1>select count(*) from leo1; 插入了230w条记录
COUNT(*)
----------------
2302656
LEO1@LEO1>create index leo1_type_name_idx on leo1(object_type,object_name);
在leo1表的object_type,object_name字段上创建复合索引，必须要创建索引，如果没有索引在进行检索时就会忽略FIRST_ROWS(n)，而使用ALL_ROWS。
Index created.
LEO1@LEO1>execute dbms_stats.gather_table_stats('LEO1','LEO1',cascade=>true); 收集统计信息
第一个LEO1指的是用户名
第二个LEO1指的是表名字
第三个cascade指的是表和表中的索引进行级联分析，如果不加只是单单对表分析
PL/SQL proceduresuccessfully completed.
LEO1@LEO1> setautotrace on; 启动执行计划
LEO1@LEO1>alter session set tracefile_identifier=optimizer; 指定trace文件标识符为optimizer
Session altered.
LEO1@LEO1>alter session set sql_trace=true; 追踪下面sql语句
Session altered.
#######################################################################################
ALL_ROWS工作模式
LEO1@LEO1>select /*+ all_rows */ * from
(select /*+ all_rows*/ l.*,rownum from
(select /*+ all_rows */ object_id,object_name,object_type,ownerfrom leo1 where object_type='TABLE' order by object_name) l
where rownum<=10)
whererownum>=1; 2 3 4 5
条件查询10条记录，使用ALL_ROWS模式
OBJECT_ID OBJECT_NAME OBJECT_TYPE OWNER ROWNUM
-------------------------------------------------- --------------- ------ ----------
73465 A TABLE LEO1 1
73465 A TABLE LEO1 2
73465 A TABLE LEO1 3
73465 A TABLE LEO1 4
73465 A TABLE LEO1 5
73465 A TABLE LEO1 6
73465 A TABLE LEO1 7
73465 A TABLE LEO1 8
73465 A TABLE LEO1 9
73465 A TABLE LEO1 10
10 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value:1112449198
-------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 1200 | | 10106 (1)| 00:02:02 |
| 1| COUNT | | | | | | |
|* 2 | FILTER | | | | | | |
| 3 | VIEW | | 10 | 1200 | | 10106 (1)| 00:02:02 |
|* 4 | COUNT STOPKEY | | | | | | |
| 5| VIEW | | 85990 | 8985K| | 10106 (1)| 00:02:02 |
|* 6 | SORT ORDER BY STOPKEY| |85990 | 3778K| 5072K| 10106 (1)| 00:02:02 |
|* 7 | TABLE ACCESS FULL | LEO1 | 85990 | 3778K| | 9126 (1)| 00:01:50 |
-------------------------------------------------------------------------------------------
采用了全表扫描方式访问数据
PredicateInformation (identified by operation id):
---------------------------------------------------
2 - filter(ROWNUM>=1)
4 - filter(ROWNUM<=10)
6 - filter(ROWNUM<=10)
7 - filter("OBJECT_TYPE"='TABLE')
Statistics
----------------------------------------------------------
106 recursive calls
0 db block gets
32818 consistent gets 32818个块一致性读，因为需要处理完所有的数据才返回给用户
32793 physical reads
0 redo size
962 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
4 sorts (memory)
0 sorts (disk)
10 rows processed
TRACE文件的内容
********************************************************************************
select /*+all_rows */ * from
(select /*+ all_rows */ l.*,rownum from
(select /*+ all_rows */object_id,object_name,object_type,owner from leo1 where object_type='TABLE'order by object_name) l
where rownum<=10)
where rownum>=1
call count cpu elapsed disk query current rows
------------- -------- ---------- -------------------- ---------- ----------------------------------------------------------
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 2 0.33 0.51 32793 32804 0 10
------------- -------- ---------- -------------------- ---------- ----------------------------------------------------------
total 4 0.33 0.51 32793 32804 0 10
Misses in librarycache during parse: 0
Optimizer mode: ALL_ROWS 模式正确，一致性读与上面一样
Parsing user id:85
Rows Row Source Operation
------- ---------------------------------------------------
10 COUNT (cr=32804 pr=32793 pw=0time=0 us)
10 FILTER (cr=32804 pr=32793 pw=0time=0 us)
10 VIEW (cr=32804 pr=32793 pw=0time=198 us cost=10106 size=1200 card=10)
10 COUNT STOPKEY (cr=32804 pr=32793 pw=0 time=72 us)
10 VIEW (cr=32804 pr=32793 pw=0time=0 us cost=10106 size=9200930 card=85990)
10 SORT ORDER BY STOPKEY (cr=32804 pr=32793 pw=0 time=0 us cost=10106size=3869550 card=85990)
89792 TABLE ACCESS FULL LEO1 (cr=32804 pr=32793 pw=0 time=2061611 us cost=9126size=3869550 card=85990)
********************************************************************************
#######################################################################################
FIRST_ROWS工作模式
LEO1@LEO1>select /*+ first_rows(10) */ * from
(select /*+ first_rows(10)*/ l.*,rownum from
(select /*+ first_rows(10) */object_id,object_name,object_type,owner from leo1 where object_type='TABLE'order by object_name) l
where rownum<=10)
whererownum>=1; 2 3 4 5
条件查询10条记录，使用FIRST_ROWS(n)模式
OBJECT_ID OBJECT_NAME OBJECT_TYPE OWNER ROWNUM
-------------------------------------------------- --------------- ------ ----------
73465 A TABLE LEO1 1
73465 A TABLE LEO1 2
73465 A TABLE LEO1 3
73465 A TABLE LEO1 4
73465 A TABLE LEO1 5
73465 A TABLE LEO1 6
73465 A TABLE LEO1 7
73465 A TABLE LEO1 8
73465 A TABLE LEO1 9
73465 A TABLE LEO1 10
10 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value:1323255736
-------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 1200 | 14 (0)| 00:00:01 |
| 1 | COUNT | | | | | |
|* 2 | FILTER | | | | | |
| 3 | VIEW | | 10 | 1200 | 14 (0)| 00:00:01 |
|* 4 | COUNT STOPKEY | | | | | |
| 5 | VIEW | | 10| 1070 | 14 (0)| 00:00:01 |
| 6 | TABLE ACCESS BY INDEX ROWID| LEO1 | 10 | 450 | 14 (0)| 00:00:01 |
|* 7 | INDEX RANGE SCAN |LEO1_TYPE_NAME_IDX | | | 3 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------------
采用了索引扫描方式访问数据，索引比全表扫描要快很多
PredicateInformation (identified by operation id):
---------------------------------------------------
2 - filter(ROWNUM>=1)
4 - filter(ROWNUM<=10)
7 - access("OBJECT_TYPE"='TABLE')
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
14 consistent gets 14个块一致性读，因为只需要优先返回前10条记录即可
0 physical reads
0 redo size
962 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10 rows processed
TRACE文件的内容
********************************************************************************
select /*+first_rows(10) */ * from
(select /*+ first_rows(10) */ l.*,rownumfrom
(select /*+ first_rows(10) */object_id,object_name,object_type,owner from leo1 where object_type='TABLE'order by object_name) l
where rownum<=10)
where rownum>=1
call count cpu elapsed disk query current rows
------------- -------- ---------- ---------- -------------------- -------------------------------------------------------
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 2 0.00 0.00 0 14 0 10
------------- -------- ---------- -------------------- ---------- -------------------------------------------------------
total 4 0.00 0.00 0 14 0 10
Misses in librarycache during parse: 1
Optimizer mode: FIRST_ROWS
Parsing user id:85
Rows Row Source Operation
------- ---------------------------------------------------
10 COUNT (cr=14 pr=0 pw=0 time=0 us)
10 FILTER (cr=14 pr=0 pw=0 time=0us)
10 VIEW (cr=14 pr=0 pw=0 time=360 uscost=14 size=1200 card=10)
10 COUNT STOPKEY (cr=14 pr=0 pw=0 time=261 us)
10 VIEW (cr=14 pr=0 pw=0 time=0 uscost=14 size=1070 card=10)
10 TABLE ACCESS BY INDEX ROWID LEO1 (cr=14 pr=0 pw=0 time=0 us cost=14size=450 card=10)
10 INDEX RANGE SCAN LEO1_TYPE_NAME_IDX (cr=4 pr=0 pw=0 time=0 us cost=3size=0 card=0)(object id 73578)
********************************************************************************
LEO1@LEO1>alter session set sql_trace=false; 关闭sql的trace功能
Session altered.
LEO1@LEO1> setautotrace off; 关闭执行计划
[oracle@leonarding1trace]$ pwd
/u01/app/oracle/diag/rdbms/leo1/LEO1/trace
[oracle@leonarding1trace]$ tkprof LEO1_ora_12962_OPTIMIZER.trc optimizer1.log sys=no
对trace文件格式化，不输出sys用户trace信息（例如 递归语句的信息）
TKPROF: Release11.2.0.1.0 - Development on Thu Dec 13 20:40:38 2012
Copyright (c)1982, 2009, Oracle and/or its affiliates. All rights reserved.
-rw-r--r-- 1oracle oinstall 7671 Dec 13 20:19optimizer1.log
[oracle@leonarding1trace]$ ll -lrt
-rw-r--r-- 1oracle oinstall 27793 Dec 13 20:40optimizer1.log
[oracle@leonarding1trace]$ vim optimizer1.log 查看trace文件内容（放在上面了）
小结：2条SQL返回相同的记录，但FIRST_ROWS要比ALL_ROWS效率高很多，因为为了最快速度返回需要的数据，只进行了14个一致性读，后面的数据还没有处理完，前面的数据就返回给用户了。而ALL_ROWS需要全表扫描所有数据块才返回结果。ALL_ROWS在OLAP系统中使用比较多。
Leonarding
2012.12.12
天津&winter
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Blog：www.leonarding.com