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78f4ee385f
count(col), unlike count(*), does not count rows for which col is NULL. However, if col's data type is not a scalar (e.g. a collection, tuple, or user-defined type) it behaves like count(*), counting NULLs too. The cause is that get_dynamic_aggregate() converts count() to the count(*) version. It works for scalars because get_dynamic_aggregate() intentionally fails to match scalar arguments, and functions::get() then matches the arguments against the pre-declared count functions. As we can only pre-declare count(scalar) (there's an infinite number of non-scalar types), we change the approach to be the same as min/max: we make count() a generic function. In fact count(col) is much better as a generic function, as it only examines its input to see if it is NULL. A unit test is added. It passes with Cassandra as well. Fixes #14198. Closes #14199