Re: OUTER JOIN with filter

On Sun, 2 Mar 2003, Tom Lane wrote:

> Stephan Szabo <sszabo@megazone23.bigpanda.com> writes:
> > On Sat, 1 Mar 2003, Nicolas Fertig wrote:
> >> Can anyone help me with the problem bellow ?
> >>
> >> SELECT TM.some_field, TS.name
> >> FROM table_main TM
> >> LEFT OUTER JOIN (SELECT name FROM table_slave WHERE c1 = 'myvalue') TS
> >> USING(id)
>
> > select tm.some_field, ts.name
> > from table_main tm left outer join table_slave ts
> > on (tm.id=ts.id and ts.c1='myvalue');
>
> Offhand I believe that these queries should generate identical plans.
> They do not at the moment --- the second one generates a worse plan
> (sorry Stephan ;-)) --- because the planner does not realize it could

I wasn't really sure if it would or not (wrote it without testing on a
running server), actually I didn't realize it wouldn't push down, and
figured it'd give pretty much the same plan, but it is less verbose. :)

> push down the ts.c1='myvalue' JOIN condition into the scan of ts, even
> though the join is OUTER.  But AFAICS it would not change the results to
> do so; ts rows failing ts.c1='myvalue' will not produce join output
> anyway, but would allow outer-joined lefthand rows to be produced.  (Can
> anyone see a hole in that logic?  It's on my to-do list to change it...)

I don't see an obvious case where it'd be different either.