Date: Thu, 17 Aug 2006 22:21:31 -0400
From: mark@mark.mielke.cc
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
Subject: Re: Q: Performance of join vs embedded query for simple queries?
Message-ID: <20060818022131.GA15599@mark.mielke.cc>
References: <20060818003326.GA11682@mark.mielke.cc>
	<1484.1155864093@sss.pgh.pa.us>
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On Thu, Aug 17, 2006 at 09:21:33PM -0400, Tom Lane wrote:
> mark@mark.mielke.cc writes:
> > I have two simple queries that do what I believe to be the exact same
> > thing.
> These are actually not equivalent per spec.
> ...
> This still isn't equivalent to the join: it'll return at most one copy
> of any sm_change row, whereas you can get multiple copies of the same
> sm_change row from the join, if there were multiple matching sm_system
> rows.  (Hm, given the unique index on (system_dbid, uid), I guess that
> couldn't actually happen --- but you have to reason about it knowing
> that that index is there, it's not obvious from the form of the query.)

> Anyway: given the way that the planner works, the IN form and the join
> form will probably take comparable amounts of time to plan.  The "=
> subselect" form is much more constrained in terms of the number of
> alternative implementations we have, so it doesn't surprise me that it
> takes less time to plan.

That makes sense. Would it be reasonable for the planner to eliminate
plan considerations based on the existence of unique indexes, or is
this a fundamentally difficult thing to get right in the general case?

I did the elimination in my head, which is why I considered the plans to
be the same. Can the planner do it?

Sub-millisecond planning/execution for simple queries on moderate
hardware seems sexy... :-)

Thanks,
mark

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