count(*)、count(val)和count(1)的解释
一、关于count的一些谣言：
1、count(*)比count(val)更慢！项目组必须用count(val)，不准用count(*)，谁用扣谁钱！
2、count(*)用不到索引，count(val)才能用到。
3、count(*)是统计出全表的记录，是吞吐量的操作，肯定用不到索引。
4、count(1)比count(*)的速度快。
二、验证count(*)和count(val)
1、首先创建一个表，使用count(*)和count(val)查询比较：
----删除echo表----
SQL> drop table echo purge;
drop table echo purge
*
第 1 行出现错误:
ORA-00942: 表或视图不存在 ----创建一张echo的测试表----
SQL> create table echo as select * from dba_objects; 表已创建。 SQL> update echo set object_id = rownum; 已更新72509行。 SQL> commit; 提交完成。 SQL> set timing on
SQL> set linesize 100
SQL> set autotrace on
SQL> select count(*) from echo; COUNT(*)
----------
72509 已用时间: 00: 00: 00.01 执行计划
----------------------------------------------------------
Plan hash value: 99109176 -------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 290 (1)| 00:00:04 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| ECHO | 80064 | 290 (1)| 00:00:04 |
------------------------------------------------------------------- Note
-----
- dynamic sampling used for this statement (level=2) 统计信息
----------------------------------------------------------
4 recursive calls
0 db block gets
1265 consistent gets
0 physical reads
11060 redo size
425 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed SQL> select count(*) from echo; COUNT(*)
----------
72509 已用时间: 00: 00: 00.01 执行计划
----------------------------------------------------------
Plan hash value: 99109176 -------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 290 (1)| 00:00:04 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| ECHO | 80064 | 290 (1)| 00:00:04 |
------------------------------------------------------------------- Note
-----
- dynamic sampling used for this statement (level=2) 统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
1038 consistent gets
0 physical reads
0 redo size
425 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed SQL> select count(object_id) from echo; COUNT(OBJECT_ID)
----------------
72509 已用时间: 00: 00: 00.01 执行计划
----------------------------------------------------------
Plan hash value: 99109176 ---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 290 (1)| 00:00:04 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | TABLE ACCESS FULL| ECHO | 80064 | 1016K| 290 (1)| 00:00:04 |
--------------------------------------------------------------------------- Note
-----
- dynamic sampling used for this statement (level=2) 统计信息
----------------------------------------------------------
4 recursive calls
0 db block gets
1112 consistent gets
0 physical reads
0 redo size
433 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed SQL> select count(object_id) from echo; COUNT(OBJECT_ID)
----------------
72509 已用时间: 00: 00: 00.01 执行计划
----------------------------------------------------------
Plan hash value: 99109176 ---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 290 (1)| 00:00:04 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | TABLE ACCESS FULL| ECHO | 80064 | 1016K| 290 (1)| 00:00:04 |
--------------------------------------------------------------------------- Note
-----
- dynamic sampling used for this statement (level=2) 统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
1038 consistent gets
0 physical reads
0 redo size
433 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
从上面的执行计划可以看出count(*)和count(val)是一样快的。
2、建立索引做比较
SQL> create index idx_object_id on echo(object_id); 索引已创建。 已用时间: 00: 00: 05.69
SQL> select count(*) from echo; COUNT(*)
----------
72509 已用时间: 00: 00: 00.05 执行计划
----------------------------------------------------------
Plan hash value: 99109176 -------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 290 (1)| 00:00:04 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| ECHO | 80064 | 290 (1)| 00:00:04 |
------------------------------------------------------------------- Note
-----
- dynamic sampling used for this statement (level=2) 统计信息
----------------------------------------------------------
5 recursive calls
0 db block gets
1113 consistent gets
0 physical reads
0 redo size
425 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed SQL> select count(object_id) from echo; COUNT(OBJECT_ID)
----------------
72509 已用时间: 00: 00: 00.08 执行计划
----------------------------------------------------------
Plan hash value: 1131838604 ---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 13 | 49 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | INDEX FAST FULL SCAN| IDX_OBJECT_ID | 80064 | 1016K| 49 (0)| 00:00:01 |
--------------------------------------------------------------------------------------- Note
-----
- dynamic sampling used for this statement (level=2) 统计信息
----------------------------------------------------------
4 recursive calls
0 db block gets
244 consistent gets
161 physical reads
0 redo size
433 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
哇，原来真的是用count(val)比count(*)要快啊，因为count(*)不能用到索引，而count(val)可以，真相真是如此吗？
3、将object_id设置为非空
SQL> alter table echo modify object_id not null; 表已更改。 已用时间: 00: 00: 01.41 SQL> select count(*) from echo; COUNT(*)
----------
72509 已用时间: 00: 00: 00.00 执行计划
----------------------------------------------------------
Plan hash value: 1131838604 -------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 49 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FAST FULL SCAN| IDX_OBJECT_ID | 80064 | 49 (0)| 00:00:01 |
------------------------------------------------------------------------------- Note
-----
- dynamic sampling used for this statement (level=2) 统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
169 consistent gets
0 physical reads
0 redo size
425 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed SQL> select count(object_id) from echo; COUNT(OBJECT_ID)
----------------
72509 已用时间: 00: 00: 00.01 执行计划
----------------------------------------------------------
Plan hash value: 1131838604 -------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 49 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | INDEX FAST FULL SCAN| IDX_OBJECT_ID | 80064 | 49 (0)| 00:00:01 |
------------------------------------------------------------------------------- Note
-----
- dynamic sampling used for this statement (level=2) 统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
169 consistent gets
0 physical reads
0 redo size
433 bytes sent via SQL*Net to client
415 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--看来count(val)和count(*)其实一样快，如果索引列是非空的，count(*)可用到索引，此时一样快！真相真是如此吗？
其实两者根本没有可比性，性能比较首先考虑写法等价，这两个语句根本就不等价。
结论：
其实优化器里的算法是这么玩的，列的偏移量决定性能，列越靠后，访问的开销越大。由于count(*)的算法与列偏移量无关，所以count(*)最快，count(最后列val)最慢。
3、用实验验证上面的结论：
SQL> set serveroutput on
SQL> set echo on
SQL> drop table t purge; 表已删除。
----构造出有25个字段的表T----
DECLARE
l_sql VARCHAR2(32767);
BEGIN
l_sql := 'CREATE TABLE t (';
FOR i IN 1..25
LOOP
l_sql := l_sql || 'n' || i || ' NUMBER,';
END LOOP;
l_sql := l_sql || 'pad VARCHAR2(1000)) PCTFREE 10';
EXECUTE IMMEDIATE l_sql;
END;
12 / PL/SQL 过程已成功完成。
----将记录还有这个表T中填充----
DECLARE
l_sql VARCHAR2(32767);
BEGIN
l_sql := 'INSERT INTO t SELECT ';
FOR i IN 1..25
LOOP
l_sql := l_sql || '0,';
END LOOP;
l_sql := l_sql || 'NULL FROM dual CONNECT BY level <= 10000';
EXECUTE IMMEDIATE l_sql;
COMMIT;
END;
13 / PL/SQL 过程已成功完成。 execute dbms_stats.gather_table_stats(ownname=>user, tabname=>'t')
SELECT num_rows, blocks FROM user_tables WHERE table_name = 'T'; PL/SQL 过程已成功完成。 SQL>
NUM_ROWS BLOCKS
---------- ----------
10000 80
----以下动作观察执行速度，比较发现count(*)最快，count(最大列)最慢----
SQL> DECLARE
l_dummy PLS_INTEGER;
l_start PLS_INTEGER;
l_stop PLS_INTEGER;
l_sql VARCHAR2(100);
BEGIN
l_start := dbms_utility.get_time;
FOR j IN 1..1000
LOOP
EXECUTE IMMEDIATE 'SELECT count(*) FROM t' INTO l_dummy;
END LOOP;
l_stop := dbms_utility.get_time;
dbms_output.put_line((l_stop-l_start)/100);
14
FOR i IN 1..25
LOOP
l_sql := 'SELECT count(n' || i || ') FROM t';
l_start := dbms_utility.get_time;
FOR j IN 1..1000
LOOP
EXECUTE IMMEDIATE l_sql INTO l_dummy;
END LOOP;
l_stop := dbms_utility.get_time;
dbms_output.put_line((l_stop-l_start)/100);
END LOOP;
END;
27 /
.18
.33
.39
.38
.42
.4
.45
.49
.48
.46
.48
.48
.55
.51
.56
.57
.61
.62
.75
.67
.68
.7
.73
.78
.77
.81
PL/SQL 过程已成功完成。
三、验证count(*)和count(1)
沿用TOM大师的解释：
count(*)、count(val)和count(1)的解释的更多相关文章
- SQLSERVER 里SELECT COUNT(1) 和SELECT COUNT(*)哪个性能好？
SQLSERVER 里SELECT COUNT(1) 和SELECT COUNT(*)哪个性能好? 今天遇到某人在我以前写的一篇文章里问到 如果统计信息没来得及更新的话,那岂不是统计出来的数据时错误的 ...
- COUNT(*),count(1),COUNT(ALL expression)，COUNT(DISTINCT expression)
创建一个测试表 IF OBJECT_ID( 'dbo.T1' , 'U' )IS NOT NULL BEGIN DROP TABLE dbo.T1; END; GO )); GO INSERT INT ...
- hql中不能写count(1)能够写count(a.id)
hql中不能写count(1)能够写count(a.id)里面写详细的属性 String hql="select new com.haiyisoft.vo.entity.cc.repo.Bu ...
- count（*）、count（1）和count（列名）的区别
count(*).count(1)和count(列名)的区别 1.执行效果上: l count(*)包括了所有的列,相当于行数,在统计结果的时候,不会忽略列值为NULL l count(1)包 ...
- 今天犯了个小错误：_dataArray.count>1 和_dataArray.count>0搞混淆了
_dataArray.count>1 和_dataArray.count>0搞混淆了:当数据为一条时,条件不成立.应该_dataArray.count>＝1 或者>0 ( ...
- COUNT(*)、COUNT(主键)、COUNT(1)
MyISAM引擎,记录数是结构的一部分,已存cache在内存中; InnoDB引擎,需要重新计算,id是主键的话,会加快扫描速度: 所以select count(*) MyISAM完胜! MyISA ...
- 【优化】COUNT(1)、COUNT(*)、COUNT(常量)、COUNT(主键)、COUNT(ROWID)、COUNT(非空列)、COUNT(允许为空列)、COUNT(DISTINCT 列名)
[优化]COUNT(1).COUNT(*).COUNT(常量).COUNT(主键).COUNT(ROWID).COUNT(非空列).COUNT(允许为空列).COUNT(DISTINCT 列名) 1. ...
- oracle count（*） 和count（列）性能
一直以为oracle中count(列)比count(*) 快,这篇文件解释了一下: http://blog.csdn.net/szstephenzhou/article/details/8446481
- COUNT(*),count(1),COUNT(ALL expression)，COUNT(DISTINCT expression) BY Group by
select column_2,count(column_2) as 'count(column_2)' ,count(column_1) as 'count(column_1)' ,count(*) ...
随机推荐
- SQLSERVER 数据库查看各表的记录数
select a.name as 表名,max(b.rows) as 记录条数 from sysobjects a ,sysindexes b where a. ...
- 缩进, Tab 还是空格?（转）
圣战 圣战个毛线 Android 好还是 iOS 好,Windows 好还是 Mac 好,编辑器好还是 IDE好,中划线好还是下划线好…写代码这么多年,会遇到很多奇葩的毫无意义的所谓脑残粉的争论 昨天 ...
- play项目部署
首先对于现场的数据库,一定要谨慎谨慎再谨慎,特别是保存有重要数据的. 使用expdp命令导入数据库: 事前准备: 1.确保linux服务器上已经正确安装oracle (10g以上版本) 2.有Xshe ...
- mysql高负载的问题排查
http://dngood.blog.51cto.com/446195/1150031 log_slow_queries = /usr/local/mysql/var/slow_queries.log ...
- 【转】Java多线程编程中易混淆的3个关键字( volatile、ThreadLocal、synchronized)总结
概述 最近在看<ThinKing In Java>,看到多线程章节时觉得有一些概念比较容易混淆有必要总结一下,虽然都不是新的东西,不过还是蛮重要,很基本的,在开发或阅读源码中经常会遇到,在 ...
- leetcode 116- Populating Next Right Pointers in Each Node
题目: Given a binary tree struct TreeLinkNode { TreeLinkNode *left; TreeLinkNode *right; TreeLinkNode ...
- MJRefresh简单处理
//下拉刷新 默认 self.bottomTableVeiw.header = [MJRefreshNormalHeader headerWithRefreshingBlock:^{ [self he ...
- Java基础(39):数据的四舍五入、去整、产生随机数---Math类的应用
使用 Math 类操作数据 Math 类位于 java.lang 包中,包含用于执行基本数学运算的方法, Math 类的所有方法都是静态方法,所以使用该类中的方法时,可以直接使用类名.方法名,如: M ...
- c++之路进阶——bzoj3343（教主的魔法）
F.A.Qs Home Discuss ProblemSet Status Ranklist Contest ModifyUser gryz2016 Logout 捐赠本站 Notice:由于本OJ ...
- SpringMvc项目分析
首先在配置文件中配置一个视图解析器,视图解析器规定了视图解析的规则,即controller处理请求之后,返回给DispatcheServlet一个字符串(也可能是ModelAndView之类的),而D ...