--//前几天测试11g Query Result Cache RC Latches时,链接http://blog.itpub.net/267265/viewspace-2632907/
--//有网友指出测试有问题,建立索引唯一,并不会导致select count(*) from t,选择索引执行.实际上执行计划还是全表扫描.
--//也就有了如下测试,不过结果有点让我吃惊,设置not null反而更慢.通过测试说明:
--//另外我也做了10g下的测试,链接如下:http://blog.itpub.net/267265/viewspace-2636321/ => [20190215]那个更快(10g).txt
1.环境:
SCOTT@book> @ 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
create table t as select rownum id from dual ;
--//分析表略.
--//另外说明一下,先建立表主要避免编译过程时报错.
2.建立测试环境:
create table job_times (sid number, time_ela number,method varchar2(20));
CREATE OR REPLACE PROCEDURE do_work
(
p_iterations IN NUMBER
,p_method IN VARCHAR2
)
IS
l_rowid ROWID;
v_t NUMBER;
BEGIN
INSERT INTO job_times VALUES ( SYS_CONTEXT (‘userenv‘, ‘sid‘) ,DBMS_UTILITY.get_time ,p_method) RETURNING ROWID INTO l_rowid;
FOR i IN 1 .. p_iterations
LOOP
SELECT COUNT (*) INTO v_t FROM t;
END LOOP;
UPDATE job_times SET time_ela = DBMS_UTILITY.get_time - time_ela WHERE ROWID = l_rowid;
COMMIT;
END;
/
CREATE OR REPLACE PROCEDURE do_work1
(
p_iterations IN NUMBER
,p_method IN VARCHAR2
)
IS
l_rowid ROWID;
v_t NUMBER;
BEGIN
INSERT INTO job_times VALUES ( SYS_CONTEXT (‘userenv‘, ‘sid‘) ,DBMS_UTILITY.get_time ,p_method) RETURNING ROWID INTO l_rowid;
FOR i IN 1 .. p_iterations
LOOP
SELECT COUNT (*) INTO v_t FROM t where id=1;
END LOOP;
UPDATE job_times SET time_ela = DBMS_UTILITY.get_time - time_ela WHERE ROWID = l_rowid;
COMMIT;
END;
/
3.测试:
--//执行脚本如下:注一定要等N个会话执行完成在回车,进行下一项测试.
--//可以打开另外的会话执行select method,count(*),avg(TIME_ELA),sum(TIME_ELA) from job_times group by method order by 3 ;
--//确定测试是否完成.
$ cat bb.txt
delete from job_times;
commit ;
drop table t purge;
create table t as select rownum id from dual ;
execute sys.dbms_stats.gather_table_stats ( OwnName => user,TabName => ‘t‘,Estimate_Percent => NULL,Method_Opt => ‘FOR ALL COLUMNS SIZE 1 ‘,Cascade => True ,No_Invalidate => false);
alter procedure do_work compile ;
alter procedure do_work1 compile ;
host sleep 5
host seq &&1 | xargs -I{} echo "sqlplus -s -l scott/&&2 <<< \"execute do_work(&&3,‘null‘)\" & " | bash > /dev/null
host read -p ‘wait finish...‘
create unique index pk_t on t(id);
alter table t modify (id not null);
host seq &&1 | xargs -I{} echo "sqlplus -s -l scott/&&2 <<< \"execute do_work(&&3,‘notnull‘)\" & " | bash > /dev/null
host read -p ‘wait finish...‘
host seq &&1 | xargs -I{} echo "sqlplus -s -l scott/&&2 <<< \"execute do_work1(&&3,‘id=1_unique_index‘)\" & " | bash > /dev/null
host read -p ‘wait finish...‘
drop index pk_t ;
create index pk_t on t(id);
host seq &&1 | xargs -I{} echo "sqlplus -s -l scott/&&2 <<< \"execute do_work1(&&3,‘id=1_index‘)\" & " | bash > /dev/null
host read -p ‘wait finish...‘
alter table t result_cache (mode force);
host seq &&1| xargs -I{} echo "sqlplus -s -l scott/&&2 <<< \"execute do_work(&&3,‘result_cache‘)\" & " | bash > /dev/null
host read -p ‘wait finish...‘
--//简单说明:执行需要3个参数,参数1:启动连接数,参数2:scott口令,参数3,循环次数.
--//执行如下: @ bb.txt 50 book 1e6
--//第1种方式:执行计划是全表扫描,逻辑读2(10g下这里是3),看到的等待事件是cursor: pin S.很奇怪11g下看不到latch: cache buffers chains相关等待事件.
--//第2种方式:建立唯一索引,加入约束id not null,这样执行计划INDEX FULL SCAN,逻辑读1.看到的等待事件是latch: cache buffers chains,偶尔能看到cursor: pin S.
--//第3种方式:执行语句加入谓词id=1,这样执行计划INDEX UNIQUE SCAN,逻辑读1.看到的等待事件是cursor: pin S,在11g下latch: cache buffers chains看不到.
注:在这种情况cbc latch减少一半比其它方式.
--//第4种方式:索引修改非唯一,执行语句加入谓词id=1,这样执行计划是INDEX RANGE SCAN,逻辑读1.看到的等待事件是latch: cache buffers chains.偶尔能看到cursor: pin S.
--//第5种方式:设置result_cache=force;逻辑读0,看到的等待事件是cursor: pin S.
--//测试结果如下:
SCOTT@book> select method,count(*),round(avg(TIME_ELA),0),sum(TIME_ELA) from job_times group by method order by 3 ;
METHOD COUNT(*) ROUND(AVG(TIME_ELA),0) SUM(TIME_ELA)
-------------------- ---------- ---------------------- -------------
result_cache 50 8611 430536
id=1_unique_index 50 9494 474714
null 50 10664 533197
id=1_index 50 28160 1407987
notnull 50 29279 1463928
--//你可以发现结果按照快慢排序 result_cache => id=1_unique_index => null => id=1_index,notnull,实际上最后2个结果很接近.
--//使用result_cache 最快很好理解,为什么设置列NULL比not null快许多呢?
--//而且设置字段id NULL是全表扫描,至少2个逻辑读(对于ctas建立的表),而设置字段id NOT NULL 走的是快速全索引扫描(1个逻辑读).
--//看测试结果 not null的情况下几乎慢了3倍.
--//实际上字段设置 not null更慢.因为这时出现cursor: pin S 外,还出现外还大量出现 latch: cache buffers chains,而全表扫描
--//反而不出现latch: cache buffers chains等待事件.这样设置not null反而更慢.
--//11g在处理latch: cache buffers chains上做了一些优化,读读情况下有时候看不到cbc latch.
--//不过id=1_unique_index这样的情况下反而比10g执行要慢.
--//在10g下测试如下:
SCOTT@test> select method,count(*),round(avg(TIME_ELA),0),sum(TIME_ELA) from job_times group by method order by 3 ;
METHOD COUNT(*) ROUND(AVG(TIME_ELA),0) SUM(TIME_ELA)
-------------------- ---------- ---------------------- -------------
id=1_unique_index 50 4864 243192
notnull 50 34134 1706713
id=1_index 50 34703 1735173
null 50 37234 1861717
--//实际上10g,11g是硬件配置一样,os安装也一样.
--//附上监测wait脚本:
$ cat wait.sql
select p1raw,p2raw,p3raw,p1,p2,p3,sid,serial#,seq#,event,status,state,wait_time_micro,seconds_in_wait,wait_class
from v$session where ( wait_class<>‘Idle‘ or (status=‘ACTIVE‘ and STATE=‘WAITED KNOWN TIME‘))
and sid not in (select sid from v$mystat where rownum=1)
order by event ;
原文地址：https://www.cnblogs.com/lfree/p/10399988.html