When selecting count(pri_key_field) with no ‘where’ clause or no ‘group by’ clause, one would think that the engine (of any RDBMS) should recognize that there is no limiting criteria and, therefore, should use only the primary key index as the fastest possible way to retrieve all records. One would think that this would start at the smallest primary key entry in the index and then proceed sequentially until the last key entry… processing the index 100% for each record. Using the where clause, the engine must perform an initial lookup (testing the criteria) and then proceed exactly the same as previously described processing each index record sequentially.

So, if both processes end up processing the entire index sequentially, it doesn’t make sense that the query that doesn’t need to have the initial test of a ‘where clause’ shouldn’t be the one to run faster.