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Re: Q on views and performance

From: "Kynn Jones" <kynnjo(at)gmail(dot)com>
To: "Dean Gibson (DB Administrator)" <postgresql(at)ultimeth(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Q on views and performance
Date: 2008-02-23 13:59:35
Message-ID: c2350ba40802230559l280eab73k8e79a8bda7a4994a@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Fri, Feb 22, 2008 at 8:48 PM, Dean Gibson (DB Administrator) <
postgresql(at)ultimeth(dot)com> wrote:

> On 2008-02-22 12:49, Kynn Jones wrote:
> > Of course, I expect that using views V<int1> and V<int2>... would
> > result in a loss in performance relative to a version that used bona
> > fide tables T<int1> and T<int2>.  My question is, how can I minimize
> > this performance loss?
>
> That used to be my thoughts too, but I have found over the years that
> the PostgreSQL execution planner is able to "flatten" SELECTs using
> VIEWs, ALMOST ALWAYS in a way that does not adversely affect
> performance, and often gives an IMPROVEMENT in performance, probably
> because by using VIEWs I am stating the query problem in a better way
> than if I try to guess the best way to optimize a SELECT.
>
> I have at least a 10:1 ratio of VIEWs to TABLEs.  Occasionally, with
> some query that is slow, I will try to rewrite it without VIEWs.  This
> ALMOST NEVER results in an improvement in performance, and when it does,
> I am able to find another way to write the VIEW and SELECT to recapture
> the gain.


Since you have experience working with views, let me ask you this.  The
converse strategy to the one I described originally would be to create the
individual tables T1, T2, T3, ..., T100, but instead of keeping around the
original (and now redundant) table T, replace it with a view V made up of
the union of T1, T2, T3, ..., T100.  The problem with this alternative is
that one cannot index V, or define a primary key constraint for it, because
it's a view.  This means that a search in V, even for a primary key value,
would be *have to be* very inefficient (i.e. I don't see how even the very
clever PostgreSQL implementers could get around this one!), because the
engine would have to search *all* the underlying tables, T1 through T100,
even if it found the desired record in T1, since it has no way of knowing
that the value is unique all across V.

Is there a way around this?

kynn

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