Re: Using Expanded Objects other than Arrays from plpgsql - Mailing list pgsql-hackers

I wrote:
> ... I'm still writing up
> details, but right now I'm envisioning completely separate sets of
> rules for the prosupport case versus the no-prosupport case.

So here is the design I've come up with for optimizing R/W expanded
object updates in plpgsql without any special knowledge from a
prosupport function.  AFAICS this requires no assumptions at all
about the behavior of called functions, other than the bare minimum
"you can't corrupt the object to the point where it wouldn't be
cleanly free-able".  In particular that means it can work for
user-written called functions in plpgsql, SQL, or whatever, not
only for C-coded functions.

There are two requirements to apply the optimization:

* If the assignment statement is within a BEGIN ... EXCEPTION block,
its target variable must be declared inside the most-closely-nested
such block.  This ensures that if an error is thrown from within the
assignment statement's expression, we do not care about the value
of the target variable, except to the extent of being able to clean
it up.

While I've not tried to write any code yet, I think both of these
conditions should be reasonably easy to verify.

Given that those conditions are met and the current value of the
assignment target variable is a R/W expanded pointer, we can
execute the assignment as follows:

So, while this design greatly expands the set of cases we can
optimize, it does lose some cases that the old approach could
support.  I envision addressing that by allowing a prosupport
function attached to the RHS' topmost function to "bless"
other cases as safe, using reasoning similar to the old rules.
(Or different rules, even, but it's on the prosupport function
to be sure it's safe.)  I don't have a detailed design in mind,
but I'm thinking along the lines of just passing the whole RHS
expression to the prosupport function and letting it decide
what's safe.  In any case, we don't need to even call the
prosupport function unless there's an exception block or
multiple RHS references to the target variable.

                        regards, tom lane