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
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Plan hash value: 2966233522
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| 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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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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