Reply-To: jesper.pedersen@redhat.com
Subject: Re: Index Skip Scan
To: Peter Geoghegan <pg@bowt.ie>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>,
 Bhushan Uparkar <bhushan.uparkar@gmail.com>,
 pgsql-hackers <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: <9f2a0339-7d4a-e632-2ad6-237b24a95fa6@redhat.com>
Date: Fri, 17 Aug 2018 13:15:46 -0400
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Hi Peter,

On 08/16/2018 03:48 PM, Peter Geoghegan wrote:
> On Wed, Aug 15, 2018 at 11:22 PM, Thomas Munro
> <thomas.munro@enterprisedb.com> wrote:
>> * groups and certain aggregates (MIN() and MAX() of suffix index
>> columns within each group)
>> * index scans where the scan key doesn't include the leading columns
>> (but you expect there to be sufficiently few values)
>> * merge joins (possibly the trickiest and maybe out of range)
> 
> FWIW, I suspect that we're going to have the biggest problems in the
> optimizer. It's not as if ndistinct is in any way reliable. That may
> matter more on average than it has with other path types.
> 

Thanks for sharing this; it is very useful to know.

Best regards,
  Jesper