Re: pgstat: add pgstat_prep_pending() for entry ref pending setup

> On Wed, Jul 15, 2026 at 03:50:19PM -0500, Sami Imseih wrote:
> > pgstat_prep_pending_entry() bundles three operations in a single call:
> > lookup, entry creation, and pending data setup. This is convenient for
> > the common case, but some callers need to do some other things in
> > between these steps. For example, looking at what needs to be done
> > for moving pg_stat_statements [1] to the cumulative statistics collector,
> > it needs to:
> >
> > 1. Look up the entry (pgstat_get_entry_ref with create=false)
> > 2. If not found, check capacity and possibly evict before creating
> > 3. Create the entry (pgstat_get_entry_ref with create=true)
> > 4. Attach pending data
> >
> > Steps 1-3 are already possible with pgstat_get_entry_ref(), but step 4
> > has no API.
> >
> > The attached adds pgstat_prep_pending() which exposes step 4 as a
> > function. Together with pgstat_get_entry_ref(), callers can now perform
> > the same work as pgstat_prep_pending_entry() in individual steps.
> > pgstat_prep_pending_entry() itself is refactored to use the new
> > function internally.
>
> Hmm. So we have the APIs to do steps 1 to 3, with an eviction cleanup
> happening in-between. Your argument is that if you would like to do
> something between steps 3 and 4, as we don't have an API to attach
> pending data. If there is nothing to do between steps 3 and 4, we can
> just:
> - use pgstat_get_entry_ref(create=false)
> - eviction
> - pgstat_prep_pending_entry() (has pgstat_get_entry_ref(create=true))
>
> So my question is: what do you intend to do between steps 3 and 4?
> More waiting for some of parallel eviction done in step 2?

What I am saying is in order to perform steps 1-4 with individual steps
rather than the single pgstat_prep_pending_entry(), the API for step 4,
to prepare the pending entry, is still missing.

What I am saying is that in order to perform steps 1-4 with individual
steps rather than the single pgstat_prep_pending_entry(), the API for
step 4, to prepare the pending entry, is missing.

In the common fast path case (step 1 succeeds, entry already exists), I have
a valid entry_ref from pgstat_get_entry_ref(create=false). To attach
pending data to it without the proposed API, I would have to call
pgstat_prep_pending_entry(), which internally calls pgstat_get_entry_ref
(create=true) again. That repeats all the work that was already done in
step 1, key initialization, setup checks, the GC scan
(pgstat_need_entry_refs_gc),
and the local cache lookup, all to get back the same entry I already hold.

pgstat_prep_pending() avoids that: it attaches pending data directly
to the entry_ref I already have, with none of the redundant overhead.

--
Sami