Message-ID: <1115315884.427a5eacd3ca9@webmail.telus.net>
Date: Thu,  5 May 2005 10:58:04 -0700
From: a3a18850@telus.net
To: pgsql-sql@postgresql.org
Subject: Re: HELP: aggregating insert rule for multirow inserts.
References: <1115234855.4279222707401@webmail.telus.net>
	<45caccdc050505003936efa435@mail.gmail.com>
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Quoting Mikey <mikeboscia@gmail.com>:

> What about this as a rule.  It seems to work for me.
> 
> create or replace rule critter_counter as
> on INSERT to critter do (
> 
>   insert into zoostats
>       select distinct new.legs
>       where new.legs not in (select legs from zoostats);
> 
>        update zoostats set headcount = (
>                select count(*)
>                from critter
>                where critter.legs = zoostats.legs
>        ) where zoostats.legs = new.legs
> );

...
Thanks, sorry I didn't mention, I was looking for an efficient solution,
given that (I thought) using a RULE that processes the whole (in-memory) NEW set
in one operation is faster than a trigger that increments each headcount for
each inserted row. It comes from the way of thinking when you work with
Sybase/MSSQL triggers, and their INSERTED and DELETED pseudotables --- what you
would get in FOR-EACH-STATEMENT triggers that implement NEW_TABLE AS and
OLD_TABLE AS.

Yes, recalculating each affected leg-count group from scratch will work.
But in the above, adding one 4legged animal requires rescan of a large number of
animals ...

Hope the zoo metaphor isn't getting a bit tedious, folks.

> On 5/4/05, Mischa Sandberg <mischa.sandberg@telus.net> wrote:
> > I'm having a problem with the use of the NEW rowset,
> > in a rule intended to aggregate across inserts.
> > 
> > I've never really grokked how NEW and OLD really work,
> > syntactically, other than that they seem to be implicit
> > in every top-level FROM clause, and any mention elsewhere
> > gets an error: '42P01: relation "*NEW*" does not exist'.
> > 
> > I've tried different flavours of the UPDATE command,
> > in the following rule, and they either produce syntax errors
> > or the wrong results.
> > 
> > Any suggestions much appreciated ...
> > 
> > ====================== CODE
> > "How many critters are in the zoo, of the 4,5,6...-legged varieties?"
> > 
> > create table critter(name text, legs int);
> > create table zoostats(legs int, headcount int default 0,
> >                       primary key(legs));
> > 
> > create or replace rule critter_counter as
> > on INSERT to critter do (
> > 
> >     insert into zoostats
> >         select distinct new.legs
> >         where new.legs not in (select legs from zoostats);
> > 
> >     update zoostats
> >     set    headcount = headcount + (select count(*)) -- "from new"
> >     where new.legs = zoostats.legs
> > );
> > 
> > insert into critter values('cat',4);
> > insert into critter values('starfish',5);
> > insert into critter values('ant',6);
> > insert into critter values('dog',4);
> > 
> > insert into critter select * from critter; -- double everything.
> > 
> > select * from zoostats;
> > 
> > drop table zoostats cascade;
> > drop table critter;
> > ====================== EXPECTED OUTPUT
> > legs headcount
> > ---- ---------
> >    4         4
> >    5         2
> >    6         2
> > ====================== ACTUAL OUTPUT
> > legs headcount
> > ---- ---------
> >    4         3      -- !?
> >    5         2
> >    6         2