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From: Melanie Plageman <[email protected]>
To: Chao Li <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Andrey Borodin <[email protected]>
Cc: Kirill Reshke <[email protected]>
Cc: Xuneng Zhou <[email protected]>
Cc: Robert Haas <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Heikki Linnakangas <[email protected]>
Subject: Re: eliminate xl_heap_visible to reduce WAL (and eventually set VM on-access)
Date: Tue, 3 Mar 2026 10:52:34 -0500
Message-ID: <CAAKRu_a7ByG2LipFADR7UYnLP4BWQKOV1sajqJC4=R37iO05+A@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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On Tue, Mar 3, 2026 at 2:33 AM Chao Li <[email protected]> wrote:
>
> 2 - 0003 - Does it make sense to also do the same renaming in PruneFreezeResult?

I could do that. Later commits remove them, so I thought it didn't
make sense. If only this commit goes in though, it would make sense.

> -                * Calculate what the snapshot conflict horizon should be for a record
> -                * freezing tuples. We can use the visibility_cutoff_xid as our cutoff
> -                * for conflicts when the whole page is eligible to become all-frozen
> -                * in the VM once we're done with it. Otherwise, we generate a
> -                * conservative cutoff by stepping back from OldestXmin.
> -                */
> -               if (prstate->set_all_frozen)
> -                       prstate->frz_conflict_horizon = prstate->visibility_cutoff_xid;
> -               else
> -               {
> -                       /* Avoids false conflicts when hot_standby_feedback in use */
> -                       prstate->frz_conflict_horizon = prstate->cutoffs->OldestXmin;
> -                       TransactionIdRetreat(prstate->frz_conflict_horizon);
> -               }
> +               Assert(TransactionIdPrecedesOrEquals(prstate->pagefrz.FreezePageConflictXid,
> +                                                                                        prstate->cutoffs->OldestXmin));
> ```
>
> At this point of Assert, can prstate->pagefrz.FreezePageConflictXid be InvalidTransactionId? My understanding is no, in that case, would it make sense to also Assert(prstate->pagefrz.FreezePageConflictXid != InvalidTransactionId)?

I think it is possible if we are doing some kind of freezing to a
multixact that we reach here and FreezePageConflictXid is
InvalidTransactionId.

> Otherwise, if prstate->pagefrz.FreezePageConflictXid is still possibly be InvalidTransactionId, then the Assert should be changed to something like:
>
> Assert(prstate->pagefrz.FreezePageConflictXid == InvalidTransactionId ||
>   TransactionIdPrecedesOrEquals(prstate->pagefrz.FreezePageConflictXid, prstate->cutoffs->OldestXmin)

This is covered by TransactionIdPrecedesOrEquals because
InvalidTransactionId is 0. We assume that in many places throughout
the code.

> I will continue with 0005 tomorrow.

Thanks for the review!

I noticed a serious bug in v35-0017: I pass hscan->modifies_base_rel
to heap_page_prune_opt() as rel_read_only, which is the opposite of
what I want to do -- it should be !hscan->modifies_base_rel. I'm going
to wait to fix it though and post a new v36 once I've batched up more
fixups.

- Melanie

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