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Oracle之count(1) count(*) count(col)区别
时间：2017-07-17 来源：
count(1)和count(*)没有任何区别(执行计划和统计信息)，而且都是统计所有行
count(col)统计col列不为空的记录，如果有索引，不管col是否为空，都能走索引，没有索引就无法走
测试
hr@ORCL> select count(1) from t;
Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 159 (2)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T | 50356 | 159 (2)| 00:00:02 |
-------------------------------------------------------------------
hr@ORCL> select count(*) from t;
Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522
-------------------------------------------------------------------
| Id | Operation | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 159 (2)| 00:00:02 |
| 1 | SORT AGGREGATE | | 1 | | |
| 2 | TABLE ACCESS FULL| T | 50356 | 159 (2)| 00:00:02 |
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