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From: Tom Lane <[email protected]>
To: Michel Pelletier <[email protected]>
Cc: [email protected]
Subject: Re: Using Expanded Objects other than Arrays from plpgsql
Date: Wed, 23 Oct 2024 12:04:30 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
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I wrote:
> One idea I was toying with is that it doesn't matter if f()
> throws an error so long as the plpgsql function is not executing
> within an exception block: if the error propagates out of the plpgsql
> function then we no longer care about the value of the variable.
> That would very substantially weaken the requirements on how f()
> is implemented.

The more I think about this idea the better I like it.  We can
improve on the original concept a bit: the assignment can be
within an exception block so long as the target variable is too.
For example, consider

	DECLARE x float8[];
	BEGIN
	   ...
	   DECLARE y float8[];
	   BEGIN
	      x := array_append(x, 42);
	      y := array_append(y, 42);
	   END;
	EXCEPTION WHEN ...;
	END;

Currently, both calls of array_append are subject to R/W optimization,
so that array_append must provide a strong guarantee that it won't
throw an error after it's begun to change the R/W object.  If we
redefine things so that the optimization is applied only to "y",
then AFAICS we need nothing from array_append.  It only has to be
sure it doesn't corrupt the object so badly that it can't be freed
... but that requirement exists already, for anything dealing with
expanded objects.  So this would put us in a situation where we
could apply the optimization by default, which'd be a huge win.

There is an exception: if we are considering

	      x := array_cat(x, x);

then I don't think we can optimize because of the aliasing problem
I mentioned before.  So there'd have to be a restriction that the
target variable is mentioned only once in the function's arguments.
For stuff like your vxm() function, that'd be annoying.  But functions
that need that and are willing to deal with the aliasing hazard could
still provide a prosupport function that promises it's okay.  What
we'd accomplish is that a large fraction of interesting functions
could get the benefit without having to create a prosupport function,
which is a win all around.

Also worth noting: in the above example, we could optimize the
update on "x" too, if we know that "x" is not referenced in the
block's EXCEPTION handlers.  I wouldn't bother with this in the
first version, but it might be worth doing later.

So if we go this way, the universe of functions that can benefit
from the optimization enlarges considerably, and the risk of bugs
that break the optimization drops considerably.  The cost is that
some cases that were optimized before now will not be.  But I
suspect that plpgsql functions where this optimization is key
probably don't contain EXCEPTION handlers at all, so that they
won't notice any change.

Thoughts?

			regards, tom lane

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