Re: making EXPLAIN extensible

On Fri, 28 Feb 2025 at 20:26, Robert Haas <robertmhaas@gmail.com> wrote:
>
> Prior to PostgreSQL 10, EXPLAIN had just 2 options: VACUUM and
> ANALYZE.

I think you meant "some time prior to PostgreSQL 10".
PostgreSQL 9.0 had 5 options, of which COSTS, BUFFERS, and FORMAT were
newly added, so only before 9.0 we had 2 options.
PostgreSQL 9.2 then added TIMING on top of that, for a total of 6
options prior to PostgreSQL 10.

> It wouldn't make sense for core to have an EXPLAIN
> option whose whole purpose is to cater to the needs of some extension,
> so that made me think of providing some extensibility infrastructure.

+1, Neon would greatly appreciate infrastructure to allow extending EXPLAIN.

> 0001 allows a loadable module to register new EXPLAIN options.
> Currently, EXPLAIN (FUNGUS) will error out, but if you want to make it
> work, this patch is for you. This patch also allows you to stash some
> state related to your new option, or options, in the ExplainState.
> Core options have hard-coded structure members; e.g. EXPLAIN (BUFFERS)
> sets es->buffers. If you add EXPLAIN (FUNGUS), there won't be an
> es->fungus, but you can get about the same effect using the new
> facilities provided here.

Does this work with parallel workers' stats?
I can't seem to figure out whether or where parallel workers would
pass through their extended explain statistics, mostly because of the
per-backend nature of ID generation making the pointers of
ExplainState->extension_state unshareable.

Kind regards,

Matthias van de Meent