Horton

Re: Add 64-bit XIDs into PostgreSQL 15

Pavel Borisov <pashkin.elfe@gmail.com>

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From: Pavel Borisov <pashkin.elfe@gmail.com>
To: Aleksander Alekseev <aleksander@timescale.com>
Cc: Postgres hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-01-28T14:43:06Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Add SLRU tests for 64-bit page case

  2. Make use FullTransactionId in 2PC filenames

  3. Use larger segment file names for pg_notify

  4. Index SLRUs by 64-bit integers rather than by 32-bit integers

>
> Did you consider using 4 bytes for pd_xid_base and another 4 bytes for
> (pd_xid_base/pd_multi_base)? This would allow calculating XMIN/XMAX
> as:
>
> XMIN = (t_min_extra_bits << 32) | t_xmin
> XMAX = (t_max_extra_bits << 32) | t_xmax
>
> ... and save 8 extra bytes in the pd_special area. Or maybe I'm
> missing some context here?
>
Hi, Alexander!

In current design it is not possible, as pd_xid_base is roughly just a
minimum 64-xid of all tuples that may fit this page. So we do not make any
extra guess that it should be in multiples of 2^32.

If we make pd_xid_base in multiples of 2^32 then after current XID crosses
the border of 2^32 then pages that contains tuples with XMIN/XMAX before
this point are not suitable for tuple inserts anymore. In effect we will
then have "sets" of the pages for each 2^32 "epoch" with freed space that
can not be used anymore.

I think it's too big a loss for gain of just 8 bytes per page.

Thank you for your dive into this matter!

-- 
Best regards,
Pavel Borisov

Postgres Professional: http://postgrespro.com <http://www.postgrespro.com>