Re: eliminate xl_heap_visible to reduce WAL (and eventually set VM on-access)

On Sat, Dec 13, 2025 at 8:59 AM Peter Eisentraut <peter@eisentraut.org> wrote:
>
> On 20.11.25 18:19, Melanie Plageman wrote:
> > +     prstate->deadoffsets = (OffsetNumber *) presult->deadoffsets;
>
> In your patch
> v22-0001-Split-heap_page_prune_and_freeze-into-helpers.patch, the
> assignment above casts away the const qualification of the function
> argument presult:

Yea, this code (prune_freeze_setup() with a const-qualified
PruneFreezeResult parameter) is actually already in master -- not just
in this patchset.

> +static void
> +prune_freeze_setup(PruneFreezeParams *params,
> +                                  TransactionId new_relfrozen_xid,
> +                                  MultiXactId new_relmin_mxid,
> +                                  const PruneFreezeResult *presult,
> +                                  PruneState *prstate)
>
> (The cast is otherwise unnecessary, since the underlying type is the
> same on both sides.)
>
> Since prstate->deadoffsets is in fact later modified, this makes the
> original const qualification invalid.

I didn't realize I was misusing const here. What I meant to indicate
by defining the prune_freeze_setup() parameter, as const, is that the
PruneFreezeResult wouldn't be modified by prune_freeze_setup(). I did
not mean to indicate that no members of PruneFreezeResult would ever
be modified. deadoffsets is not modified in prune_freeze_setup(). So,
are you saying that I can't define a parameter as const if even the
caller modifies it?

I'm fine with committing a change, I just want to understand.

- Melanie