Re: WIP patch for multiple column assignment in UPDATE

On Thu, Jun 19, 2014 at 9:37 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Pavel Stehule <pavel.stehule@gmail.com> writes:
>> I did some tests and It looks so it allows only some form of nested loop.
>
> [ shrug... ]  It's a subplan.  One evaluation per outer row is what
> people are expecting.

Is it theoretically possible to convert a construct like this to a
semi-join?  I notice we don't, even when this new syntax isn't used:

rhaas=# explain select a, (select b from bar where foo.a = bar.a) from foo;                        QUERY PLAN
------------------------------------------------------------Seq Scan on foo  (cost=0.00..855145.00 rows=10000 width=4)
SubPlan1    ->  Seq Scan on bar  (cost=0.00..85.50 rows=1 width=4)          Filter: (foo.a = a)Planning time: 0.078 ms
 

...but I'm wondering if that's an unimplemented feature or if there's
some reason why it's a bad idea.

-- 
Robert Haas
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