Horton

Re: GiST VACUUM

Heikki Linnakangas <hlinnaka@iki.fi>

View thread View on postgr.es
From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Andrey Borodin <x4mmm@yandex-team.ru>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>, Robert Haas <robertmhaas@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>, Костя Кузнецов <chapaev28@ya.ru>
Date: 2018-08-05T11:18:59Z
Lists: pgsql-hackers 
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.

> 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.

- Heikki

Commits

  1. Use full 64-bit XID for checking if a deleted GiST page is old enough.

  2. Refactor checks for deleted GiST pages.

  3. Delete empty pages during GiST VACUUM.

  4. Scan GiST indexes in physical order during VACUUM.

  5. Prevent GIN deleted pages from being reclaimed too early