Differences and performance of count (1) count (*) count (col)

Analysis of the differences between count (1) count (*) count (col) and performance count (1) and count ), in addition, the count (col) statistics for all rows are performed for records whose col columns are not empty. If an index exists, indexes can be used regardless of whether col is empty, without an index, you cannot test www.2cto.com [SQL] hr @ ORCL> select count (1) from t; Execution Plan -------------------------------------------------------- Plan hash value: 2966233522 bytes | 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 | explain hr @ ORCL> select count (*) from t; Execution Plan ---------------------------------------------------------- Plan hash value: 2966233522 bytes | 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 | -------------------------------------------------------------------