Horton

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

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: John Naylor <johncnaylorls@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>, Nathan Bossart <nathandbossart@gmail.com>, Andres Freund <andres@anarazel.de>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Yura Sokolov <y.sokolov@postgrespro.ru>
Date: 2024-03-25T01:02:18Z
Lists: pgsql-hackers

Attachments

On Mon, Mar 25, 2024 at 1:53 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> John Naylor <johncnaylorls@gmail.com> writes:
> > Done. I pushed this with a few last-minute cosmetic adjustments. This
> > has been a very long time coming, but we're finally in the home
> > stretch!

Thank you for the report.

>
> I'm not sure why it took a couple weeks for Coverity to notice
> ee1b30f12, but it saw it today, and it's not happy:

Hmm, I've also done Coverity Scan in development but I wasn't able to
see this one for some reason...

>
> /srv/coverity/git/pgsql-git/postgresql/src/include/lib/radixtree.h: 1621 in local_ts_extend_down()
> 1615                    node = child;
> 1616                    shift -= RT_SPAN;
> 1617            }
> 1618
> 1619            /* Reserve slot for the value. */
> 1620            n4 = (RT_NODE_4 *) node.local;
> >>>     CID 1594658:  Integer handling issues  (BAD_SHIFT)
> >>>     In expression "key >> shift", shifting by a negative amount has undefined behavior.  The shift amount, "shift", is as little as -7.
> 1621            n4->chunks[0] = RT_GET_KEY_CHUNK(key, shift);
> 1622            n4->base.count = 1;
> 1623
> 1624            return &n4->children[0];
> 1625     }
> 1626
>
> I think the point here is that if you start with an arbitrary
> non-negative shift value, the preceding loop may in fact decrement it
> down to something less than zero before exiting, in which case we
> would indeed have trouble.  I suspect that the code is making
> undocumented assumptions about the possible initial values of shift.
> Maybe some Asserts would be good?  Also, if we're effectively assuming
> that shift must be exactly zero here, why not let the compiler
> hard-code that?
>
> -       n4->chunks[0] = RT_GET_KEY_CHUNK(key, shift);
> +       n4->chunks[0] = RT_GET_KEY_CHUNK(key, 0);

Sounds like a good solution. I've attached the patch for that.

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.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