From: Tom Lane <tgl@sss.pgh.pa.us>
To: Julius Tuskenis <julius@nsoft.lt>
Cc: Merlin Moncure <mmoncure@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: unexpected stable function behavior
Date: Mon, 14 Mar 2011 13:17:33 -0400
Message-ID: <12872.1300123053@sss.pgh.pa.us> (raw)
In-Reply-To: <4D7DD5CF.4080502@nsoft.lt>
References: <4D78FB98.7090609@nsoft.lt>
<AANLkTimTR_f98H4+h57iW6-S=BjZaMTSQ6VznRgXU5Ly@mail.gmail.com>
<4D7DD5CF.4080502@nsoft.lt>
Julius Tuskenis <julius@nsoft.lt> writes:
> From the postgresql documentation about STABLE functions: "This
> category allows the optimizer to optimize multiple calls of the function
> to a single call." I thought that this means that optimizer executes the
> function only for now parameter sets and stores results in some "cache"
> and use it if the parameters are already known.
No, it does not. That function property *allows* the optimizer to
invoke the function fewer times than would happen in an un-optimized
query. It does not *require* it to do so. There is no such cache
mechanism in Postgres, and it's unlikely that there ever will be,
because it probably would be a net performance loss on average.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, julius@nsoft.lt, mmoncure@gmail.com
Subject: Re: unexpected stable function behavior
In-Reply-To: <12872.1300123053@sss.pgh.pa.us>
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