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From: Heikki Linnakangas <hlinnakangas@vmware.com>
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Tomas Vondra <tv@fuzzy.cz>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: GIN improvements part 1: additional information
Date: Mon, 16 Dec 2013 13:30:13 +0200
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On 12/12/2013 06:44 PM, Alexander Korotkov wrote:
> I've thought about different algorithms little more. General problem I see
> is online update. We need it while it is typically not covered by
> researches at all. We already have to invent small index in the end of
> page. Different encoding methods adds more challenges. In general, methods
> can be classified in two groups:
> 1) Values aren't aligned by bytes (gamma-codes, PFOR etc.)
> 2) Multiple values are packed together in small group (simple-9, simple-18)

Ok.

> For the first group of methods when inserting in the middle of the page we
> would have to do not byte-aligned shift of right part of values. I don't
> know how expensive is this shift but I expect that it would be much slower
> than memmove.

Agreed.

> When values are packed into small groups, we have to either insert
> inefficiently encoded value or re-encode whole right part of values.

It would probably be simplest to store newly inserted items 
uncompressed, in a separate area in the page. For example, grow the list 
of uncompressed items downwards from pg_upper, and the compressed items 
upwards from pg_lower. When the page fills up, re-encode the whole page.

- Heikki


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