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Re: Improving our clauseless-join heuristics

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Improving our clauseless-join heuristics
Date: 2012-04-13 16:42:23
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Lists: pgsql-hackers
On Fri, Apr 13, 2012 at 10:51 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Fri, Apr 13, 2012 at 10:32 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> After some reflection I think that the blame should be pinned on
>>> have_relevant_joinclause(), which is essentially defined as "is there
>>> any join clause that can be evaluated at the join of these two
>>> relations?".  I think it would work better to define it as "is there any
>>> join clause that both these relations participate in?".
>> I think it's getting a little late in the day to be whacking the
>> planner around too much, but I have to admit that seems like a pretty
>> good and safe change to me, so maybe we should go ahead and do it.
>> I'm a bit worried, though, that with all the planner changes this
>> release we are going to spend a lot of time tracking down regressions
>> either in planning time or in plan quality.
> Could be.  I think though that this fits in pretty naturally with the
> parameterized-path changes, since both of them are really directed
> towards being able to apply inner indexscans in cases where we could
> not before.

Yeah, I had the same thought.  I am more worried that either the
changes we made for index-only scans or the parameterized paths stuff
is going to turn out to introduce nasty, as-yet-unforeseen
regressions.  But at this point we're in for that pain whatever we do
about this.

Robert Haas
The Enterprise PostgreSQL Company

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