Re: Using SETOF in plpgsql function - Mailing list pgsql-sql

Tom Lane wrote:
> Jan Wieck <janwieck@Yahoo.com> writes:
> > hlefebvre wrote:
> >> I'd like to return a set of integer in an pl/pgsql function. How can I
> >> do that ?
>
> >     You  can't. Not with PL/pgSQL nor with any other PL or C. The
> >     problem is nested deeper and requires the  planned  querytree
> >     redesign to get solved.
>
> Not really.  Coincidentally enough, I am just in the middle of removing
> execQual.c's hard-wired assumption that only SQL-language functions
> can return sets.  (This is a side effect of fixing the function manager
> so that SQL functions can be called in all contexts, eg used as index
> functions.)  If you want to fix plpgsql so that it retains state and
> can produce multiple elements of a set over repeated calls, the same
> way that SQL functions do, then it could be done today.
   Not   that   easy.  PL/pgSQL  isn't  a  state  machine.   The   precompiled code is kind of a nested tree of
statements.  A   RETURN causes a controlled return() through all nested levels   of the PL executors C calls. This
mightclose  SPI  calls  in   execution as well. Imagine a code construct like
 
       FOR rec IN SELECT * FROM customer LOOP           RETURN rec.cust_id AND RESUME;       END LOOP;
   which  would  be  the correct syntax for returning sets. What   happens in PL/pgSQL while execution is, that at the
beginning  of  the  loop the SPI query for SELECT is performed, and then   the loop executed for all rows in the SPI
resultset. And  of   course, you can have nested loops, why not.
 
   Now  you want to return the first value. If you really return   to the fmgr at this time, the connection to the  SPI
manager   must  be  closed,  loosing the result set. So how to continue   later?
 
   If we want to make it now for  sets  of  scalar  values  (not   tuple sets), we could add another feature to the
fmgrand the   PL handlers, which we need later anyway.
 
   In the case of a call to a PL or C function returning a  set,   the  fmgr  creates  a temp table and calls the
functionwhich   fills the temp table with all the  return  values.  Now  fmgr   changes  the  execution  trees  func
nodein a way that it is   operating like an SQL function - holding a seqscan  over  the   temp table. After the last
resultis returned, the temp table   is removed. This'd work for tuple sets as well (so  the  temp   table then is our
tuple-source).


Jan

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