Re: [PATCH] Add transforms feature

On 06/14/2013 11:11 AM, Peter Eisentraut wrote:
> A transform is an SQL object that supplies to functions for converting
> between data types and procedural languages.  For example, a transform
> could arrange that hstore is converted to an appropriate hash or
> dictionary object in PL/Perl or PL/Python.
>
> Externally visible changes:
>
> - new SQL commands CREATE TRANSFORM and DROP TRANSFORM
>
> - system catalog pg_transform
>
> - transform support in PL/Perl and PL/Python
>
> - PL/Perl and PL/Python install their header files for use by external
>   types
I wonder if that should be extended to install headers for hstore,
ltree, and while we're at it, intarray as well?

You handle getting access to the headers by using include path flags:

PG_CPPFLAGS = -I$(top_srcdir)/src/pl/plpython $(python_includespec)
-I$(top_srcdir)/contrib/hstore

which is fine for in-tree builds but AFAIK means that pgxs cannot build
these extensions. If the extension modules installed their headers that
problem would go away; you'd just
   #include "contrib/hstore/hstore.h"

Many modules already have useful non-static functions that work with raw
types and aren't callable via the fmgr, as well as the macros and
typdefs that make working with them easier. intarray in particular would
be extremely useful to use from other extensions, but in the past I've
landed up copying it and then adding the extra functions I needed
because I couldn't access its header in a PGXS build.

Peter's already answered my main worry on this, which is whether any
platform would prevent us from doing the required runtime symbol lookup
from shared libs loaded at the same level. I knew RTLD_GLOBAL allowed it
for dlopen(...), but wasn't sure about other platforms.

Thoughts? Should contrib modules be able to install headers and use each
others' symbols? I've seen interest in this in the wild from people who
want to use hstore in their own extensions and I've had uses for it
myself with intarray.

-- Craig Ringer                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services