I've been reading past threads, studying backend code, reviewing Alex 
Pilosov's "cursor foo" patch (submitted last August/September, but never 
applied), and conversing off list with a few people regarding a possible 
implementation of Set Returning Functions (or SRF for short). Below is 
my proposal for how this might work. After discussion, and if there is 
no objection, I would like to work on this implementation with the hope 
that it could be in place for 7.3.


Proposal for set returning functions (SRF):
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The problem:
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Currently the ability to return multiple row, multiple column result 
sets from a function is quite limited. In fact, it is not possible to 
return multiple columns directly. It is possible to work around this 
limitation, but only in a clumsy way (see contrib/dblink for an 
example). Alternatively refcursors may be used, but they have their own 
set of issues, not the least of which is they cannot be used in view 
definitions or exist outside of explicit transactions.


The feature:
-----------------------------------------------------
The desired feature is the ability to return multiple row, multiple 
column result sets from a function, or set returning functions (SRF) for 
short.


Do we want this feature?
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Based on the many posts on this topic, I think the answer to this is a 
resounding yes.


How do we want the feature to behave?
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A SRF should behave similarly to any other table_ref (RangeTblEntry), 
i.e. as a tuple source in a FROM clause. Currently there are three 
primary kinds of RangeTblEntry: RTE_RELATION (ordinary relation), 
RTE_SUBQUERY (subquery in FROM), and RTE_JOIN (join). SRF would join 
this list and behave in much the same manner.


How do we want the feature implemented? (my proposal)
-----------------------------------------------------
1. Add a new table_ref node type:
    - Current nodes are RangeVar, RangeSubselect, or JoinExpr
    - Add new RangePortal node as a possible table_ref. The RangePortal
      node will be extented from the current Portal functionality.

2. Add support for three modes of operation to RangePortal:
   a. Repeated calls -- this is the existing API for SRF, but
      implemented as a tuple source instead of as an expression.
   b. Materialized results -- use a TupleStore to materialize the
      result set.
   c. Return query -- use current Portal functionality, fetch entire
      result set.

3. Add support to allow the RangePortal to materialize modes 1 and 3, if 
needed for a re-read.

4. Add a WITH keyword to CREATE FUNCTION, allowing SRF mode to be 
specified. This would default to mode a) for backward compatibility.

5. Ignore the current code which allows functions to return multiple 
results as expressions; we can leave it there, but deprecate it with the 
intention of eventual removal.

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Thoughts/comments would be much appreciated.

Thanks,

Joe


Mon, 22 Apr 2002 10:16:41 -0700
Joe Conway <mail@joeconway.com>
[RFC] Set Returning Functions