Subject: Re: Using Expanded Objects other than Arrays from plpgsql

From: Tom Lane <[email-masked]>
Date: Sun, 20 Oct 2024 13:13:31 -0400

Michel Pelletier <[email-masked]> writes:
> I found this thread from the original path implementation from Tom Lane in
> 2015:
> https://www.postgresql.org/message-id/E1Ysvgz-0000s0-DP%40gemulon.postgresql.org
>> In this initial implementation, a few heuristics have been hard-wired
>> into plpgsql to improve performance for arrays that are stored in
>> plpgsql variables. We would like to generalize those hacks so that
>> other datatypes can obtain similar improvements, but figuring out some
>> appropriate APIs is left as a task for future work.

Yeah, we thought that it wouldn't be appropriate to try to design
general APIs till we had more kinds of expandable objects.  Maybe
now is a good time to push forward on that.

> My first thought was to add a flag to CREATE TYPE like "EXPANDED = true" or
> some other better name that indicates that the object can be safely taken
> ownership of in its expanded state and not copied.

Isn't that inherent in the notion of R/W vs R/O expanded-object
pointers?

> And then there is just removing the existing restriction on arrays only.
> Is any other expanded object out there really interested in being
> flattened/expanded over and over again?

I'm not sure.  It seems certain that if the object is already expanded
(either R/W or R/O), the paths for that in plpgsql_exec_function could
be taken regardless of its specific type.  The thing that is debatable
is "if the object is in a flat state, should we forcibly expand it
here?".  That could be a win if the function later does object
accesses that would benefit --- but it might never do so.  We chose
to always expand arrays, and we've gotten little pushback on that,
but the tradeoffs might be different for other sorts of expanded
objects.

The other problem is that plpgsql only knows how to do such expansion
for arrays, and it's not obvious how to extend that part.

But it seems like we could get an easy win by adjusting
plpgsql_exec_function along the lines of

l. 549:
-                    if (!var->isnull && var->datatype->typisarray)
+                    if (!var->isnull)

l. 564:
-                        else
+                        else if (var->datatype->typisarray)

How far does that improve matters for you?

The comment above line 549 cross-references exec_assign_value,
but it looks like that's already set up to be similarly type-agnostic
about values that are already expanded.

			regards, tom lane

---

From: Michel Pelletier <[email-masked]>
Date: Sun, 20 Oct 2024 11:25:30 -0700

> now is a good time to push forward on that.

Great, I'm happy to be involved!

> Isn't that inherent in the notion of R/W vs R/O expanded-object

I'm not sure I'm qualified enough to agree with you but I see your point.

> be taken regardless of its specific type.

Agreed.

>   The thing that is debatable

The OneSparse objects will always expand themselves on first use via a
DatumGetExpandedArray like macro wrapper, there is no run-time benefit to
their flat representation, so I'm fine with not forcing their expansion
ahead of time, but once I expand it I'd like it to stay expanded until the
function returns (as much as possible) the serialization costs for very
large sparse matrices is heavy.

> for arrays, and it's not obvious how to extend that part.

Perhaps a third member function for ExpandedObjectMethods that formalizes
the expansion interface like found in DatumGetExpandedArray?  I closely
follow that same pattern in my code.

> ...

I'll give it a try in my local benchmarking code and get back to you, thank
you for the fast reply and the insight!

-Michel