Re: Postgres views cannot use both union and join/where

On Wed, Oct 20, 2021 at 6:58 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> "David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> > On Tuesday, October 19, 2021, Michael Lewis <mlewis(at)entrata(dot)com> wrote:
> >> On Tue, Oct 19, 2021 at 3:48 PM Mithran Kulasekaran <
> >> mithranakulasekaran(at)gmail(dot)com> wrote:
> >>> create view template_view (id, name, description, is_staged) as
> >>> select t.id,t.name, t.description, false as is_staged
> >>> from template t
> >>> left join template_staging ts on t.name = ts.name and ts.name is null
>
> >> Does that work? I've only seen that type of logic written as-
> >> left join template_staging ts on t.name = ts.name
> >> where ts.name is null
>
> > The are functionally equivalent, though the timing of the expression
> > evaluation differs slightly.
>
> No, not at all. Michael's version correctly implements an anti-join,
> where the first version does not. The reason is that the WHERE clause
> "sees" the column value post-JOIN, whereas the JOIN/ON clause "sees"
> values pre-JOIN.
>

Yeah, my bad. I was actually thinking this but then figured the OP
wouldn't have written an anti-join that didn't actually work.

My original email was going to be:

Adding the single table expression to the ON clause is shorthand for
writing:

SELECT t.* FROM template AS t LEFT JOIN (SELECT * FROM template_staging
WHERE template_staging.name IS NULL) AS ts ON t.name = ts.name;

David J.