Re: plan cache overhead on plpgsql expression

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2020-03-26 14:37:59 -0400, Tom Lane wrote:
>> + * This function, together with CachedPlanIsSimplyValid, provides a fast path
>> + * for revalidating "simple" generic plans. The core requirement to be simple
>> + * is that the plan must not require taking any locks, which translates to
>> + * not touching any tables; this happens to match up well with an important
>> + * use-case in PL/pgSQL.

> Hm - is there currently sufficient guarantee that we absorb sinval
> messages? Would still matter for types, functions, etc?

There are potentially issues of that sort throughout the backend, not
just here, since we don't have any locking on types or functions.
I don't think it's this patch's job to address that. In practice
I think we've thought about it and concluded that the cost/benefit
of introducing such locks just isn't promising:

* Generally speaking you can't do anything very interesting to a type
anyway, at least not with supported DDL. The worst-case situation that
could materialize AFAIK is possibly evaluating slightly-stale constraints
for a domain. (The typcache does have sinval invalidation for those
constraints, but I don't recall offhand how much we guarantee about
how quickly we'll notice updates.)

* For functions, you might execute a somewhat stale version of the
function body. The bad effects there are pretty limited since a function
is defined by just one catalog row, unlike tables, so you can't see a
self-inconsistent version of it.

The amount of lock overhead that it would take to remove those edge
cases seems slightly staggering, so I doubt we'd ever do it.

> While it'd do a small bit unnecessary work, I do wonder if it'd be
> better to use this code in ResourceOwnereReleaseInternal().

When and if we refactor to expose this sort of thing more generally,
it might be worth doing it like that. I don't think it helps much
right now.

> Perhaps add a reference to the new (appreciated, btw) DO comment above?

Can do.

Again, thanks for reviewing!

regards, tom lane