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Re: tricky query

From: "Merlin Moncure" <merlin(dot)moncure(at)rcsonline(dot)com>
To: "Bruno Wolff III" <bruno(at)wolff(dot)to>
Cc: "Postgresql Performance" <pgsql-performance(at)postgresql(dot)org>,"John A Meinel" <john(at)arbash-meinel(dot)com>, "elein" <elein(at)varlena(dot)com>
Subject: Re: tricky query
Date: 2005-06-28 19:36:29
Message-ID: 6EE64EF3AB31D5448D0007DD34EEB3415C2C07@Herge.rcsinc.local (view raw or flat)
Thread:
Lists: pgsql-performance
> On Tue, Jun 28, 2005 at 12:02:09 -0400,
>   Merlin Moncure <merlin(dot)moncure(at)rcsonline(dot)com> wrote:
> >
> > Confirmed.  Hats off to you, the above some really wicked querying.
> > IIRC I posted the same question several months ago with no response
and
> > had given up on it.  I think your solution (smallest X1 not in X) is
a
> > good candidate for general bits, so I'm passing this to varlena for
> > review :)
> >
> > SELECT t1.id+1 as id_new FROM id_test t1
> >     WHERE NOT EXISTS
> >         (SELECT t2.id FROM id_test t2 WHERE t2.id = t1.id+1)
> >     ORDER BY t1.id LIMIT 1;
> 
> You need to rework this to check to see if row '1' is missing. The
> above returns the start of the first gap after the first row that
> isn't missing.

Correct.  

In fact, I left out a detail in my original request in that I had a
starting value (easily supplied with where clause)...so what I was
really looking for was a query which started at a supplied value and
looped forwards looking for an empty slot.  John's supplied query is a
drop in replacement for a plpgsql routine which does exactly this.

The main problem with the generate_series approach is that there is no
convenient way to determine a supplied upper bound.  Also, in some
corner cases of my problem domain the performance was not good.

Merlin



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