Re: OUTER JOIN with filter

Stephan Szabo <sszabo(at)megazone23(dot)bigpanda(dot)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
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 concur with the other comments that there's nothing wrong with the
given query ... other than such trivial details as it doesn't work ...
the sub-select doesn't produce an ID column. So I think Nicolas is
giving us an over-censored description of his real problem; but without
the omitted details, we're not likely to offer much useful help.

regards, tom lane