Horton

Re: [PoC] Improve dead tuple storage for lazy vacuum

John Naylor <john.naylor@enterprisedb.com>

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From: John Naylor <john.naylor@enterprisedb.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: Nathan Bossart <nathandbossart@gmail.com>, Andres Freund <andres@anarazel.de>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Yura Sokolov <y.sokolov@postgrespro.ru>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2023-01-18T04:44:14Z
Lists: pgsql-hackers 
On Mon, Jan 16, 2023 at 3:18 PM Masahiko Sawada <sawada.mshk@gmail.com>
wrote:
>
> On Mon, Jan 16, 2023 at 2:02 PM John Naylor
> <john.naylor@enterprisedb.com> wrote:

> > + * Add Tids on a block to TidStore. The caller must ensure the offset
numbers
> > + * in 'offsets' are ordered in ascending order.
> >
> > Must? What happens otherwise?
>
> It ends up missing TIDs by overwriting the same key with different
> values. Is it better to have a bool argument, say need_sort, to sort
> the given array if the caller wants?

Now that I've studied it some more, I see what's happening: We need all
bits set in the "value" before we insert it, since it would be too
expensive to retrieve the current value, add one bit, and put it back.
Also, as a consequence of the encoding, part of the tid is in the key, and
part in the value. It makes more sense now, but it needs more than zero
comments.

As for the order, I don't think it's the responsibility of the caller to
guess if it needs sorting -- if unordered offsets lead to data loss, this
function needs to take care of it.

> > + uint64 last_key = PG_UINT64_MAX;
> >
> > I'm having some difficulty understanding this sentinel and how it's
used.
>
> Will improve the logic.

Part of the problem is the English language: "last" can mean "previous" or
"at the end", so maybe some name changes would help.

--
John Naylor
EDB: http://www.enterprisedb.com

Commits

  1. radixtree: Fix SIGSEGV at update of embeddable value to non-embeddable.

  2. Get rid of anonymous struct

  3. Teach radix tree to embed values at runtime

  4. Teach TID store to skip bitmap for small numbers of offsets

  5. Use bump context for TID bitmaps stored by vacuum

  6. Fix alignment of stack variable

  7. Use TidStore for dead tuple TIDs storage during lazy vacuum.

  8. Rethink create and attach APIs of shared TidStore.

  9. Fix inconsistent function prototypes with function definitions.

  10. Fix a calculation in TidStoreCreate().

  11. Fix potential integer handling issue in radixtree.h.

  12. Add TIDStore, to store sets of TIDs (ItemPointerData) efficiently.

  13. Fix link error for test_radixtree module on Windows

  14. Blind attempt to fix ODR violations

  15. Fix incorrect format specifier for int64

  16. Fix redefinition of typedefs

  17. Add template for adaptive radix tree

  18. Fix signedness error in 9f225e992 for gcc

  19. Introduce helper SIMD functions for small byte arrays

  20. Optimize vacuuming of relations with no indexes.

  21. Add bound check before bsearch() for performance

  22. Allocate consecutive blocks during parallel seqscans