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：