Re: best way to determine start of new statement within - Mailing list pgsql-hackers

The semantics of this conceptually is not too
bad.  The implementation could be tricky.

For any given DML or sub-DML (select, eg) the scope
should be for that DML. The DML is the "parent"
of the function.  The DML is the statement 
context and the function is the function 
context.  

statement --> function                |                + --> statement --> function
      |                                            + --> statement
 
statement --> function1, function2                |                |                |                +--> statement ->
function3               |                                            |                |
          +--> statement                |                |                |                + --> statement -->
function4                                           |                                            + --> statement
 

For example:

select myfunc() from foo; -- 1 statement

select myfunc() from  (select myfunc2() from foo);-- subselect ( select ): both actually    have the same memory
duration,but have different    statement contexts and different function contexts.-- the scope of each matches its
select

select myfunc() from foo; --where myfunc invokes select myfunc2() from bar;
The scope of myfunc is the parent select.
The scope of myfunc2 resets at each iteration of
myfunc(), but is available for each iteration of
itself.  Note that these things can nest very
deeply.

select myfunc() from foo where myfunc3() = 't';-- both myfunc() and myfunc3() have the same    statement parent.-- if
myfunc3()invoked select * from myfunc4()    then each iteration of myfunc3() resets the    statement context of that
select.

The new statement corresponds to a single invocation
of one DML. Memory should last through the whole
invocation loop.

Am I making sense here?  

Where does the function context info memory get
available for allocation and where does it
get deallocated?  (It does get deallocated
via garbage collection, right?) I think the scope
of this is correct.

We implemented this at informix and got a lot of
stuff wrong.  Memory pool hell, but we had to deal
with a multi-threaded server.  Illustra did
it right so I think postgres already has the handles to
the hook the memory into the right scope.

Do you want me to try to write this up into
more formal definitions?

elein

On Fri, Jul 25, 2003 at 08:31:14AM -0700, Joe Conway wrote:
> Tom Lane wrote:
> >Joe Conway <mail@joeconway.com> writes:
> >>Specifically I'd like to reset a variable within my PL/R interpreter 
> >>each time a new SQL statement begins.
> >
> >Define "new SQL statement".  In particular, what of a PL function
> >executing multiple SQL statements inside an outer SQL statement that
> >invoked the function?  Unless you've got a clear idea of the semantics
> >you want, it's not going to be very profitable to discuss
> >implementations ...
> >
> 
> Well, I guess in a situation like this:
> a)  SELECT * FROM myfunc1()...
> where myfunc1() executes via spi
> b)      SELECT * FROM myfunc2()...
> 
> I'd be interested in being able to detect when outer statement (a) 
> starts, as well as the fact that it is at nesting level 0. And similarly 
> it would be nice to be able to detect when statement (b) starts along 
> with the fact that it is at nesting level 1. But for the moment, I'd be 
> happy with just the former.
> 
> Joe
> 
> 
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