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From: Heikki Linnakangas <hlinnakangas@vmware.com>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Tomas Vondra <tv@fuzzy.cz>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: GIN improvements part 1: additional information
Date: Wed, 20 Nov 2013 19:02:44 +0200
Message-ID: <528CEB34.7080702@vmware.com> (raw)
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On 06.11.2013 17:36, Alvaro Herrera wrote:
> Just for my own illumination, can someone explain this bit?
>
> + If a posting list is too large to store in-line in a key entry, a posting tree
> + is created. A posting tree is a B-tree structure, where the ItemPointer is
> + used as the key. At the leaf-level, item pointers are stored compressed, in
> + "varbyte encoding".
>
> I think the first ItemPointer mentioned (the key) refers to a TID
> pointing to the index, and "item pointers stored compressed" refers to
> the TIDs pointing to the heap (the data).  Is that correct?

No, they both refer to TIDs pointing to the heap.

> I'm also interested in the "FIXME explain varbyte encoding" explanation
> currently missing, if somebody can write it down ...

Alexander's latest version filled in that explanation (haven't read it 
myself yet)

- Heikki


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