[20190215]那个更快(10g).txt

来源：这里教程网时间：2026-03-03 13:00:10 作者：

[20190215]那个更快(10g).txt --//昨天测试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. --//不过我后面的测试还是让我有点吃惊....设置not null反而更慢.. --//在10g下重复测试看看. 1.环境: SCOTT@test> @ &r/ver1 PORT_STRING VERSION BANNER ------------------------------ -------------- ---------------------------------------------------------------- x86_64/Linux 2.4.xx 10.2.0.4.0 Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi create table t as select rownum id from dual ; create unique index pk_t on t(id); --//分析表略. --//多个会话同时并发执行如下语句. --//方式一:注这个时候索引没用,因为id可以为null. select count(*) from t; --//方式二: --//加入约束:alter table t modify (id not null);也就是使用索引 alter table t modify (id not null); select count(*) from t; --//方式三(索引唯一): select count(*) from t where id=1; --//方式四:(索引不唯一) drop index pk_t; create index pk_t on t(id); select count(*) from t where id=1; --//大家认为那个更快完成或者讲花的时间更少?通过测试说明问题. 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: --//测试脚本如下,10g不支持result cache. --//执行脚本如下:注一定要等50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); hos seq &&1 | xargs -I{} echo "sqlplus -s -l scott/&&2 <<< \"execute do_work(1e6,'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(1e6,'notnull')\" & " | bash > /dev/null host read -p 'wait finish...' host seq &&1 | xargs -I{} echo "sqlplus -s -l scott/&&2 <<< \"execute do_work1(1e6,'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(1e6,'id=1_index')\" & " | bash > /dev/null host read -p 'wait finish...' --//注:参数1 启动连接数, 参数2 scott的密码. --//说明: --//第1次,id null,全表扫描T,逻辑读3个.主要等待事件是latch: cache buffers chains. --//第2次:id not null,建立唯一索引.执行计划走INDEX FULL SCAN,逻辑读1.主要等待事件是latch: cache buffers chains. --//第3次:查询增加where id=1,执行计划走 INDEX UNIQUE SCAN,逻辑读1,主要等待事件是cursor: pin S . --//第4次:查询增加where id=1,由于索引不唯一,执行计划走INDEX RANGE SCAN,逻辑读1,主要等待事件是latch: cache buffers chains. --//测试结果如下: 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 4789 239432 notnull 50 34493 1724641 id=1_index 50 34568 1728409 null 50 38319 1915970 --//第2次测试结果: 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 --//可以发现10g下not null 与id=1 索引非唯一的情况测试很接近,这个很好理解逻辑读都是1.主要等待事件都是latch: cache buffers chains. --//而null的情况全表扫描的逻辑读是3,这样发生cbc latch的等待事件更加严重，这样比逻辑读1的情况下更加慢一些。 --//而id=1 并且使用唯一索引的情况下,oracle对这样情况进行特殊优化，仅仅有1次cbc latch拴锁。主要等待事件是cursor: pin S . --//顺便贴上看等待事件的脚本(wait10g.sql). select p1raw,p2raw,p3raw,p1,p2,p3,sid,serial#,seq#,event,state,wait_time,seconds_in_wait from v$session where wait_class<>'Idle' and sid not in (select sid from v$mystat where rownum=1) order by event ; --//而11g下呢? 贴出11.2.0.4下的测试结果如下: 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 8805 440251 id=1_unique_index 50 9505 475229 null 50 10841 542033 notnull 50 28310 1415522 id=1_index 50 29531 1476574 --//你可以发现与11g很大不同,10g下id 字段 null 与id 字段 not null差别不大.两者都后大量出现latch: cache buffers chains. --//而id=1,如果看vage的书就明白,仅仅出现1次cbc latch.而且非常快. --//实际上11g的环境与10g环境服务器配置基本一样.感觉11g在latch: cache buffers chains做了一些改进,全表扫描很奇怪看不到 --//cbc latch的等待事件.但是11g虽然减少cbc latch相关等待事件时间,但是通过索引唯一读取记录的需要时间是增加的(对比上面10g的 --//测试结果),其中一些细节我自己也讲不好,就不说明了. --//另外11g下id not null,建立唯一索引.执行计划走INDEX FULL SCAN,逻辑读1.主要等待事件是latch: cache buffers chains. --//这是11g下有如下两种情况能看到cbc latch的情况(notnull,id=1_index)，尽管逻辑读为1，你可以发现是最慢的，而且时间比null --//做全表扫描(逻辑读3)的情况下还慢1倍还多。 --//顺便贴上看等待事件的脚本(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 ; --//补充11g的测试脚本如下，加入了将表设置为result_cahe=force的情况。 $ 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); hos seq &&1 | xargs -I{} echo "sqlplus -s -l scott/&&2 <<< \"execute do_work(1e6,'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(1e6,'notnull')\" & " | bash > /dev/null host read -p 'wait finish...' host seq &&1 | xargs -I{} echo "sqlplus -s -l scott/&&2 <<< \"execute do_work1(1e6,'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(1e6,'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(1e6,'result_cache')\" & " | bash > /dev/null host read -p 'wait finish...'