The cost can be calculated, but not always. Example 1: SELECT ... FROM table WHERE key BETWEEN c1 AND c2 - here we can get the number of records and bytes

The cost can be calculated, but not always. Example 1: SELECT ... FROM table WHERE key BETWEEN c1 AND c2 - here we can get the number of records and bytes to scan before even starting the query. Example 2: SELECT ... FROM table WHERE key = (SELECT ...) - here we cannot know how many records will be scanned until we will finish the subquery. Maybe some estimations can be applied... but we don't do that.