Reply-To: jesper.pedersen@redhat.com
Subject: Re: Index Skip Scan
To: Thomas Munro <thomas.munro@gmail.com>
Cc: David Rowley <david.rowley@2ndquadrant.com>,
 Dmitry Dolgov <9erthalion6@gmail.com>,
 Floris Van Nee <florisvannee@optiver.com>, James Coleman <jtc331@gmail.com>,
 Rafia Sabih <rafia.pghackers@gmail.com>, Jeff Janes <jeff.janes@gmail.com>,
 Peter Geoghegan <pg@bowt.ie>, Tomas Vondra <tomas.vondra@2ndquadrant.com>,
 Thomas Munro <thomas.munro@enterprisedb.com>,
 Bhushan Uparkar <bhushan.uparkar@gmail.com>,
 PostgreSQL-development <pgsql-hackers@postgresql.org>,
 Alexander Korotkov <a.korotkov@postgrespro.ru>
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From: Jesper Pedersen <jesper.pedersen@redhat.com>
Organization: Red Hat
Message-ID: <c9b12a76-5b95-a29d-9d02-cac74ab40253@redhat.com>
Date: Wed, 10 Jul 2019 09:10:36 -0400
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Hi,

On 7/9/19 10:14 PM, Thomas Munro wrote:
>> Thomas, do you have any ideas for this ? I can see that MySQL did the
>> functionality in two change sets (base and function support), but like
>> you said we shouldn't paint ourselves into a corner.
> 
> I think amskip() could be augmented by later patches to take a
> parameter 'skip mode' that can take values SKIP_FIRST, SKIP_LAST and
> SKIP_FIRST | SKIP_LAST (meaning please give me both).  What we have in
> the current patch is SKIP_FIRST behaviour.  Being able to choose
> SKIP_FIRST or SKIP_LAST allows you do handle i, MAX(j) GROUP BY (i)
> ORDER BY i (ie forward scan for the order, but wanting the highest key
> for each distinct prefix), and being able to choose both allows for i,
> MIN(j), MAX(j) GROUP BY i.  Earlier I thought that this scheme that
> allows you to ask for both might be incompatible with David's
> suggestion of tracking UniqueKeys in paths, but now I see that it
> isn't: it's just that SKIP_FIRST | SKIP_LAST would have no UniqueKeys
> and therefore require a regular agg on top.  I suspect that's OK: this
> min/max transformation stuff is highly specialised and can have
> whatever magic path-building logic is required in
> preprocess_minmax_aggregates().
> 

Ok, great.

Thanks for your feedback !

Best regards,
  Jesper