Re: GiST VACUUM

From: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
To: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Cc: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Костя Кузнецов <chapaev28(at)ya(dot)ru>
Subject: Re: GiST VACUUM
Date: 2018-08-05 16:45:36
Message-ID: DABC2BB9-154A-47E2-B1D9-B58A4023E6D7@yandex-team.ru
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Hi!

> 5 авг. 2018 г., в 16:18, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> написал(а):
>
> On 31/07/18 23:06, Andrey Borodin wrote:
>>> On a typical GiST index, what's the ratio of leaf vs. internal
>>> pages? Perhaps an array would indeed be better.
> >
>> Typical GiST has around 200 tuples per internal page. I've switched
>> to List since it's more efficient than bitmap.
>
> Hmm. A ListCell is 16 bytes, plus the AllocChunk header, 16 bytes. 32
> bytes per internal page in total, while a bitmap consumes one bit per page, leaf or internal. If I'm doing
> my math right, assuming the ratio of leaf pages vs internal pages is
> 1:200, a List actually consumes more memory than a bitmap; 256 bits per
> internal page, vs. 200 bits. An array, with 4 bytes per block number,
> would be the winner here.
We do not know size of this array beforehand. I can implement normal ArrayList though (with repallocing array) or linked list of chunks. Or anything from data structures zoo.
Or just stick with bitmap (my preferred way).
>
>> But I have to note that default growth strategy of Bitmap is not good: we will be repallocing byte by byte.
>
> True, that repallocing seems bad. You could force it to be pre-allocated
> by setting the last bit. Or add a function to explicitly enlarge the bitmap.
OK, I'll think of proper resize function (ensure capacity, to be precise).

Best regards, Andrey Borodin.

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