Date: Thu, 21 Feb 2013 01:25:24 +0000
From: Ben Morrow <ben@morrow.me.uk>
To: Sergey Konoplev <gray.ru@gmail.com>
Cc: pgsql-sql <pgsql-sql@postgresql.org>
Subject: Re: Volatile functions in WITH
Message-ID: <20130221012523.GD29651@anubis.morrow.me.uk>
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At 12PM -0800 on 20/02/13 you (Sergey Konoplev) wrote:
> On Wed, Feb 20, 2013 at 10:16 AM, Ben Morrow <ben@morrow.me.uk> wrote:
> >> If you got mixed up with plpgsql anyway what is the reason of making
> >> this WITH query constructions instead of implementing everything in a
> >> plpgsql trigger on DELETE on exp then?
> >
> > I'm not sure what you mean. "exp" isn't a table, it's a WITH CTE. The
> 
> Sorry, I meant "item" of course, "exp" was a typo.

OK.

> > statement is deleting some entries from "item", and replacing some of
> > them with new entries, based on the information in the "item_expired"
> > view. I can't do anything with a trigger on "item", since there are
> > other circumstances where items are deleted that shouldn't trigger
> > replacement.
> 
> Okay, I see.
> 
> If the case is specific you can make a simple plpgsql function that
> will process it like FOR _row IN DELETE ... RETORNING * LOOP ...
> RETURN NEXT _row; END LOOP;

Yes, I *know* I can write a function if I have to. I can also send the
whole lot down to the client and do the inserts from there, or use a
temporary table. I was hoping to avoid that, since the plain INSERT case
works perfectly well.

> >     select *
> >     from (select j.type, random() r from item j) i
> >     where i.type = 1
> >
> > the planner will transform it into
> >
> >     select i.type, random() r
> >     from item i
> >     where i.type = 1
> >
> > before planning, so even though random() is volatile it will only get
> > called for rows of item with type = 1.
> 
> Yes, functions are executed depending on the resulting plan "A query
> using a volatile function will re-evaluate the function at every row
> where its value is needed".
> 
> > I don't know if this happens, or may sometimes happen, or might happen
> > in the future, for rows eliminated because of DISTINCT.
> 
> It is a good point. Nothing guarantees it in a perspective. Optimizer
> guarantees a stable result but not the way it is reached.

Well, it makes functions which perform DML a lot less useful, so I
wonder whether this is intentional behaviour.

Ben


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