这两天听了将近20场演讲，感觉收获很多，最深的感觉就是自己还有很长的路要走。有几个点记录一下：
昨天听老猫讲，提到一个普遍的问题就是Oracle里count(*)、count(1)和count(主键)到底哪个快的问题。这个问题看起来很简单，每个人都会有自己的答案，去百度上搜会出来一大堆帖子来讲哪个更快。但是老猫说了它们三个其实是一样的，我听到之后也觉得挺诧异的，因为我记得别人跟我说过count(主键)会快，然后自己简单想了一下，觉得好像是那么回事的就没有深入去追究。接着老猫说官方有这样的说法这三个其实是等价的。晚上回来之后到MOS上查了一下，居然被我找到了How the Oracle CBO Chooses a Path for the SELECT COUNT(*) Command (文档 ID 124717.1)。这篇文档讲的就是在CBO优化器模式下，Oracle怎样去评估没有where条件select count(*)和select count(colum)语句的最优路径。
1、创建测试表并设计测试场景：
--创建测试表 sys@ORCL>createtablejournal_entries 2(id_jenumber(8),3date_jedatenotnull,4balancednumber,5constraintindx_ecr_id_jeprimarykey(id_je) 6); Tablecreated. --创建索引 sys@ORCL>createindexindx_ecr_date_je_balancedonjournal_entries(date_je,balanced); Indexcreated. sys@ORCL>createindexindx_ecr_balanced_date_jeonjournal_entries(balanced,date_je); Indexcreated. sys@ORCL>createindexindx_ecr_balancedonjournal_entries(balanced); Indexcreated. --插入测试数据 sys@ORCL>insertintojournal_entriesvalues(1,sysdate,11); 1rowcreated. sys@ORCL>insertintojournal_entriesvalues(2,21); 1rowcreated. sys@ORCL>insertintojournal_entriesvalues(3,31); 1rowcreated. sys@ORCL>insertintojournal_entriesvalues(4,41); 1rowcreated. sys@ORCL>insertintojournal_entriesvalues(5,51); 1rowcreated. sys@ORCL>insertintojournal_entriesvalues(6,61); 1rowcreated. sys@ORCL>insertintojournal_entriesvalues(7,71); 1rowcreated. sys@ORCL>insertintojournal_entriesvalues(8,81); 1rowcreated. sys@ORCL>insertintojournal_entriesvalues(9,91); 1rowcreated. sys@ORCL>commit; Commitcomplete. --收集统计信息 sys@ORCL>execdbms_stats.gather_table_stats(ownname=>USER,tabname=>'JOURNAL_ENTRIES',cascade=>true); PL/sqlproceduresuccessfullycompleted.
设计四个场景进行对比：
Sel2 : Select count(1) from journal_entries;
Sel3 : Select count(id_je) from journal_entries;
Sel4 : Select count(balanced) from journal_entries;
1、场景1和场景2等价
@H_404_27@For CBO,Sel1 and Sel2 are strictly equivalent
sys@ORCL>altersessionsetstatistics_level=all; Sessionaltered. sys@ORCL>selectcount(*)fromjournal_entries; COUNT(*) ---------- 9 sys@ORCL>select*fromtable(dbms_xplan.display_cursor(null,null,'runstats_last')); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ sql_ID5ja3ukp4wd73p,childnumber0 ------------------------------------- selectcount(*)fromjournal_entries Planhashvalue:42135099 --------------------------------------------------------------------------------------------- |Id|Operation|Name|Starts|E-Rows|A-Rows|A-Time|Buffers| --------------------------------------------------------------------------------------------- |0|SELECTSTATEMENT||1||1|00:00:00.01|1| |1|SORTAGGREGATE||1|1|1|00:00:00.01|1| |2|INDEXFULLSCAN|INDX_ECR_ID_JE|1|9|9|00:00:00.01|1| --------------------------------------------------------------------------------------------- 14rowsselected. sys@ORCL>selectcount(1)fromjournal_entries; COUNT(1) ---------- 9 sys@ORCL>select*fromtable(dbms_xplan.display_cursor(null,'runstats_last')); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ sql_IDgbxjjuqj9j7ww,childnumber0 ------------------------------------- selectcount(1)fromjournal_entries Planhashvalue:42135099 --------------------------------------------------------------------------------------------- |Id|Operation|Name|Starts|E-Rows|A-Rows|A-Time|Buffers| --------------------------------------------------------------------------------------------- |0|SELECTSTATEMENT||1||1|00:00:00.01|1| |1|SORTAGGREGATE||1|1|1|00:00:00.01|1| |2|INDEXFULLSCAN|INDX_ECR_ID_JE|1|9|9|00:00:00.01|1| --------------------------------------------------------------------------------------------- 14rowsselected.
@H_404_27@可以看到两个语句的执行计划是完全相同的。
2、场景3也与前两个场景等价，因为id_je有NOT NULL约束
@H_404_27@For Sel3,CBO does the same as for Sel1 and Sel2 since "id_je" has a@H_404_27@NOT NULL constraint.
sys@ORCL>selectcount(id_je)fromjournal_entries; COUNT(ID_JE) ------------ 9 sys@ORCL>select*fromtable(dbms_xplan.display_cursor(null,'runstats_last')); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ sql_IDb1p4v15dwx7hs,childnumber0 ------------------------------------- selectcount(id_je)fromjournal_entries Planhashvalue:42135099 --------------------------------------------------------------------------------------------- |Id|Operation|Name|Starts|E-Rows|A-Rows|A-Time|Buffers| --------------------------------------------------------------------------------------------- |0|SELECTSTATEMENT||1||1|00:00:00.01|1| |1|SORTAGGREGATE||1|1|1|00:00:00.01|1| |2|INDEXFULLSCAN|INDX_ECR_ID_JE|1|9|9|00:00:00.01|1| --------------------------------------------------------------------------------------------- 14rowsselected.
可以看到执行计划与前两个也是完全相同的。
4、场景4跟前边3个不同，因为balanced列上没有NOT NULL约束，但是balanced列上有索引，那会走这个列上的索引么？我们来看一下执行计划：
sys@ORCL>selectcount(balanced)fromjournal_entries; COUNT(BALANCED) --------------- 9 sys@ORCL>select*fromtable(dbms_xplan.display_cursor(null,'runstats_last')); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ sql_IDbc3bc8c0fg14z,childnumber0 ------------------------------------- selectcount(balanced)fromjournal_entries Planhashvalue:3638043346 -------------------------------------------------------------------------------------------------------- |Id|Operation|Name|Starts|E-Rows|A-Rows|A-Time|Buffers| -------------------------------------------------------------------------------------------------------- |0|SELECTSTATEMENT||1||1|00:00:00.01|1| |1|SORTAGGREGATE||1|1|1|00:00:00.01|1| |2|INDEXFULLSCAN|INDX_ECR_DATE_JE_BALANCED|1|9|9|00:00:00.01|1| -------------------------------------------------------------------------------------------------------- 14rowsselected.
我们看到这个执行计划没有走balanced列上的索引，而是走了和date_je的联合索引。这个可以查看另一篇文档：Note:67522.1 Why is my index not used?
小结一下：
我这里只是简单的从执行计划上看count(*)、count(1)和count(主键)其实是一致，MOS的文档中详细的讲解了Oracle是如何评估执行计划的，也可以使用10053 event查看CBO优化器是如何做出选择的。由于我的功力还不够，对于10053事件还不是很明白，暂时就先不做演示了，要不哪说错了就不好了，这也可以做为以后博客分享的内容。
从这个事情上来看，我们对于一件事情应该做一个深入的研究，有充足的证据来证明，尤其是想要在某一方面有深入发展的时候。
原文链接：https://www.f2er.com/oracle/211240.html