WIP: Hierarchical Queries - stage 1

Hi everyone,

After spending several days reading PostgreSQL source code (and another
couple of days coding), I've managed to come up with some alpha code
that attempts to implement non-recursive WITH common table expressions.
Having got this far, I feel that I need to ask for advice from some more
experienced PostgreSQL hackers and so I post the alpha version here.

The patch attempts to alter the parser grammar, and using code modified
from that used when processing subselects in the FROM clause, attempts
to treat the CTE as if it were presented the same as a FROM subselect.

My main issue at the moment is that the code in transformFromClauseItem
seems a terrible hack, mainly because the grammar returns each string
within the FROM clause as a RangeVar, and transformFromClauseItem
assumes that each RangeVar represents a physical relation. Of course,
this is not the case when referencing a CTE and so the code first checks
to see if an entry has already been created when processing the WITH
clause; if it does then we return NULL to indicate that
transformFromClause should do nothing. Messy, but I wanted to see what
other developers thought before jumping in and rewriting this part of
the code.

Another point to think about is what should a query return if the SELECT
doesn't refer to a CTE? For example:

- WITH foo(a, b) AS (SELECT * FROM pg_class) SELECT * FROM pg_class;

Since I've inserted the CTE as a range table entry of type RTE_SUBQUERY
then the current behaviour is to perform a cross-join between foo and
bar which I'm not sure is the correct behaviour since CTEs seem to be
more like views in this respect.

Finally, the current code fails when supplying an additional alias to a
CTE in the select statement and then trying to refer to it, e.g.

- with myrel(p1) as (select oid from pg_class) select myrel.p1 from
myrel AS foo, pg_class AS bar WHERE myrel.p1 = bar.oid; -- WORKS

- with myrel(p1) as (select oid from pg_class) select myrel.p1 from
myrel AS foo, pg_class AS bar WHERE foo.p1 = bar.oid; -- FAILS

ERROR: missing FROM-clause entry for table "foo" at character 103

So in this case, should foo be accepted as a valid alias for myrel? My
feeling is that I will end up with an RTE_CTE range table entry kind
which borrows from the current subquery behaviour, but it would be very
useful to see where the similarity between the two range table entry
types ends.

TIA,

Mark.