To: Julius Tuskenis <julius@nsoft.lt>
cc: Merlin Moncure <mmoncure@gmail.com>, pgsql-performance@postgresql.org
Subject: Re: unexpected stable function behavior 
In-reply-to: <4D7DD5CF.4080502@nsoft.lt> 
References: <4D78FB98.7090609@nsoft.lt>
	<AANLkTimTR_f98H4+h57iW6-S=BjZaMTSQ6VznRgXU5Ly@mail.gmail.com>
	<4D7DD5CF.4080502@nsoft.lt>
Comments: In-reply-to Julius Tuskenis <julius@nsoft.lt>
	message dated "Mon, 14 Mar 2011 10:46:07 +0200"
Date: Mon, 14 Mar 2011 13:17:33 -0400
Message-ID: <12872.1300123053@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

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