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

Chao Li <li.evan.chao@gmail.com>

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From: Chao Li <li.evan.chao@gmail.com>
To: Yura Sokolov <y.sokolov@postgrespro.ru>
Cc: "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>, Heikki Linnakangas <hlinnaka@iki.fi>, Andres Freund <andres@anarazel.de>, Sami Imseih <samimseih@gmail.com>
Date: 2026-02-27T07:48:17Z
Lists: pgsql-hackers 

> On Feb 27, 2026, at 00:22, Yura Sokolov <y.sokolov@postgrespro.ru> wrote:
> 
> 
> Chao Li, new access functions doen't branch because I've made separate
> functions to store at slots for prepared transactions.
> 

```
+	OldestMemberMXactId[procno - MaxChildren + MaxBackends] = mxact;
```

Wow, this is clever. It removes NUM_AUXILIARY_PROCS from procno without introducing an extra if check.

However, I am not a big fan of the new global variable names:
```
MaxChildren = MaxBackends + NUM_AUXILIARY_PROCS;
TotalProcs = MaxChildren + max_prepared_xacts;
TotalXactProcs = MaxBackends + max_prepared_xacts;
```

MaxChildren is actually the total number of backends plus auxiliary processes. Maybe something like MaxProcs would better reflect that?

TotalProcs is really the length of the PGPROC array in shared memory. Prepared transactions are not real processes; they just occupy some slots in the proc array. So perhaps a name like MaxPGProcSlots or MaxBackendAndPreparedSlots would be more accurate.

As for TotalXactProcs, maybe something like MaxBackendAndPreparedSlots would make the intent clearer.

Anyway, I am not very good at naming things. You or others may have better suggestions.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/

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