On Mon, 2021-02-01 at 19:14 -0600, Matt Zagrabelny wrote: > > > What is count(*) counting then? I thought it was rows. > > > > Yeah, but count(id) only counts rows where id isn't null. > > I guess I'm still not understanding it... > > I don't have any rows where id is null: Then the *result* of count(*) and count(id) will be the same. The asterisk in count(*) is misleading. Different from any other programming language that I know, the SQL standard has decided that you cannot have an aggregate function without arguments. You have to use the asterisk in that case. So count(*) really is count(), that is, it counts one for every row that it finds, no matter what the row contains. But count(id) includes a check: if "id IS NULL", it is not counted. If that condition is satisfied for all "id"s, you end up with the same count. But count(id) is more expensive, because it will perform this unnecessary NULLness check for each row. In short: use count(*) if you want to count rows, and use count(x) if you want to count all rows where x IS NOT NULL. Yours, Laurenz Albe -- Cybertec | https://www.cybertec-postgresql.com
