Deparsing rewritten query

Hi,

I sometimes have to deal with queries referencing multiple and/or complex
views. In such cases, it's quite troublesome to figure out what is the query
really executed. Debug_print_rewritten isn't really useful for non trivial
queries, and manually doing the view expansion isn't great either.

While not being ideal, I wouldn't mind using a custom extension for that but
this isn't an option as get_query_def() is private and isn't likely to change.

As an alternative, maybe we could expose a simple SRF that would take care of
rewriting the query and deparsing the resulting query tree(s)?

I'm attaching a POC patch for that, adding a new pg_get_query_def(text) SRF.

Usage example:

SELECT pg_get_query_def('SELECT * FROM shoe') as def;
def
--------------------------------------------------------
SELECT shoename, +
sh_avail, +
slcolor, +
slminlen, +
slminlen_cm, +
slmaxlen, +
slmaxlen_cm, +
slunit +
FROM ( SELECT sh.shoename, +
sh.sh_avail, +
sh.slcolor, +
sh.slminlen, +
(sh.slminlen * un.un_fact) AS slminlen_cm,+
sh.slmaxlen, +
(sh.slmaxlen * un.un_fact) AS slmaxlen_cm,+
sh.slunit +
FROM shoe_data sh, +
unit un +
WHERE (sh.slunit = un.un_name)) shoe; +

(1 row)