Re: Avoiding unnecessary clog lookups while freezing

On Thu, Dec 29, 2022 at 12:20 PM Peter Geoghegan <pg@bowt.ie> wrote:
> > It seems somewhat wrong that we discard all the work that
> > heap_prepare_freeze_tuple() did. Yes, we force freezing to actually happen in
> > a bunch of important cases (e.g. creating a new multixact), but even so,
> > e.g. GetMultiXactIdMembers() is awfully expensive to do for nought. Nor is
> > just creating the freeze plan free.
>
> I'm not sure what you mean by that. I believe that the page-level
> freezing changes do not allow FreezeMultiXactId() to call
> GetMultiXactIdMembers() any more often than before. Are you concerned
> about a regression, or something more general than that?

Here's an idea that seems like it could ameliorate the issue:

When we're looping through members from GetMultiXactIdMembers(), and
seeing if we can get away with !need_replace/FRM_NOOP processing, why
not also check if there are any XIDs >= OldestXmin among the members?
If not (if they're all < OldestXmin), then we should prefer to go
further with processing the Multi now -- FRM_NOOP processing isn't
actually cheaper.

We'll already know that a second pass over the multi really isn't
expensive. And that it will actually result in FRM_INVALIDATE_XMAX
processing, which is ideal. Avoiding a second pass is really just
about avoiding FRM_RETURN_IS_MULTI (and avoiding FRM_RETURN_IS_XID,
perhaps, though to a much lesser degree).

-- 
Peter Geoghegan