Re: Common Table Expressions (WITH RECURSIVE) patch

>>> >> My interpretation of 7.13: General Rules: 2.b is that it should be
>>> >> single evaluation, even if RECURSIVE is present.
>>> >>
>>> >> The previous discussion was here:
>>> >>
>>> >> http://archives.postgresql.org/pgsql-hackers/2008-07/msg01292.php
>
> I am  blind, I didn't find any reason, why materialisation isn't useable.

I believe it's because of these two (closely related) problems:

# The basic
# implementation clearly ought to be to dump the result of the subquery
# into a tuplestore and then have the upper level read out from that.
# However, we don't have any infrastructure for having multiple
# upper-level RTEs reference the same tuplestore.  (Perhaps the InitPlan
# infrastructure could be enhanced in that direction, but it's not ready
# for non-scalar outputs today.)  Also, I think we'd have to teach
# tuplestore how to support multiple readout cursors.  For example,
# consider
#     WITH foo AS (SELECT ...) SELECT ... FROM foo a, foo b WHERE ...
# If the planner chooses to do the join as a nested loop then each
# Scan node needs to keep track of its own place in the tuplestore,
# concurrently with the other node having a different place.

...Robert