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Re: Fix bug in multixact Oldest*MXactId initialization and access

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Chao Li <li.evan.chao@gmail.com>, Sami Imseih <samimseih@gmail.com>
Cc: Yura Sokolov <y.sokolov@postgrespro.ru>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2026-03-02T17:24:02Z
Lists: pgsql-hackers 
On 02/03/2026 06:08, Chao Li wrote:
>> The comment here:
>>
>> +static inline MultiXactId *
>> +PreparedXactOldestMemberMXactIdSlot(ProcNumber procno)
>> +{
>> +       /*
>> +        * The entries with indexes >= MaxBackends in the OldestMemberMXactId
>> +        * array are reserved for regular backends.
>>
>> should say "array are reserved for prepared transactions"
>>
> 
> +1
> 
> And I saw the other code comment in multixact.c around line 162 needs an update because of this patch:
> ```
> * Per-backend data starts here. We have two arrays stored in the area
> * immediately following the MultiXactStateData struct. Each is indexed by
> * ProcNumber.
> ```
> 
> It says “Each is indexed by ProcNumber”, but it’s no longer accurate for OldestMemberMXactId prepared-xact entries, which now use index (procno - FIRST_PREPARED_XACT_PROC_NUMBER).

Fixed those and some other comment work, and pushed. Thanks!

- Heikki

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 test for row-locking and multixids with prepared transactions

  2. Skip prepared_xacts test if max_prepared_transactions < 2

  3. Fix OldestMemberMXactId and OldestVisibleMXactId array usage

  4. Redefine backend ID to be an index into the proc array

  5. Fix an old bug in multixact and two-phase commit. Prepared transactions can