Patch: Allow SQL-language functions to reference parameters by parameter name

I just remembered to make time to advance this from WIP to proposed
patch this week... and then worked out I'm rudely dropping it into the
last commitfest at the last minute. :/

Anyway, my interpretation of the previous discussion is a general
consensus that permitting ambiguous parameter/column references is
somewhat unsavoury, but better than the alternatives:

http://archives.postgresql.org/pgsql-hackers/2011-04/msg00433.php
http://archives.postgresql.org/pgsql-hackers/2011-04/msg00744.php
(and surrounds)

The attached patch is essentially unchanged from my WIP version; it's
updated to current master (d0dcb31), and fixes a trivial copy/paste
error. It passes `make check`.

Also attached is a rather light doc patch, which I've separated because
I'm hesitant about which approach to take. The attached version just
changes the existing mention of naming parameters in:

http://www.postgresql.org/docs/9.1/static/xfunc-sql.html#XFUNC-NAMED-PARAMETERS

It presumably ought to be clearer about the name resolution
priorities... in a quick look, I failed to locate the corresponding
discussion of column name references to link to (beyond a terse sentence
in 4.2.1).

The alternative would be to adjust most of the examples in section 35.4,
using parameter names as the preferred option, and thus make $n "the
other way".

I'm happy to do that, but I figure it'd be a bit presumptuous to present
such a patch without some discussion; it's effectively rewriting the
project's opinion of how to reference function parameters.

With regard to the discussion about aliasing the function name when used
as a qualifier
(http://archives.postgresql.org/pgsql-hackers/2011-04/msg00871.php) my
only suggestion would be that using $0 (as in, '$0.paramname') appears
safe -- surely any spec change causing it issues would equally affect
the existing $1 etc. '$.paramname' seems to look better, but presumably
runs into trouble by looking like an operator.

That whole discussion seems above my pay grade, however.

Original WIP post:

http://archives.postgresql.org/pgsql-hackers/2011-03/msg01479.php

This is an open TODO:

http://wiki.postgresql.org/wiki/Todo#SQL-Language_Functions

I've just added the above post to the CF app; I'll update it to point to
this one once it appears.

Thanks!

Matthew

--
matthew(at)trebex(dot)net