Count(*) Question
Friends,
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?
If this is true, is there a better way to get a count of records?
Thanks,
Peter Darley
On Tue, Apr 30, 2002 at 04:45:33PM -0700, Peter Darley wrote:
Friends,
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?
Yes, I beleive so.
If this is true, is there a better way to get a count of records?
I think count(1) is the common suggestion.
--
Martijn van Oosterhout <kleptog@svana.org> http://svana.org/kleptog/
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Canada, Mexico, and Australia form the Axis of Nations That
Are Actually Quite Nice But Secretly Have Nasty Thoughts About America
"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
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 totalnumber of
records if all the fields in a record are NULL?
Yes, I beleive so.
If this is true, is there a better way to get a count of records?
I think count(1) is the common suggestion.
Interesting. In 7.3devel, it does not fail to count the completely-null rows
in count(*). Does it actually do this for any version?
"Joel Burton" <joel@joelburton.com> writes:
Interesting. In 7.3devel, it does not fail to count the completely-null rows
in count(*). Does it actually do this for any version?
ISTR that back around 6.4 we had some problems with getting the
semantics of count(*) right ... but it's been quite awhile since
count(*) and count(1) weren't exactly the same thing.
regards, tom lane