Re: Count(*) Question

"Peter Darley" <pdarley@kinesis-cem.com> writes:
>     I was reading through the Rules section of the online docs, and noticed the
> following note: (* is just an abbreviation for all the attribute names of a
> relation. It is expanded by the parser into the individual attributes, so
> the rule system never sees it.)
>     Does this mean that count(*) may return less than the total number of
> records if all the fields in a record are NULL?

No.  "SELECT * FROM" means "select all the fields available from the
FROM tables", and "SELECT foo.* FROM ..., foo, ..." means "select
all the fields available from table foo, given the other constraints
of the query".  But "SELECT count(*) FROM ..." means "count all the
records produced by this FROM-expression", as opposed to "SELECT
count(some-value) FROM ..." which means "count how many records yield
a non-null result for `some-value' in this FROM-expression".

The SQL spec writers blew it by using * to mean two different things.

PG actually translates COUNT(*) into COUNT(1).  Since 1 is never
NULL, this produces the correct result per spec.  COUNT(0), or
COUNT(any-guaranteed-not-null-expression), would produce the same
answer.

            regards, tom lane