From: Tom Lane <tgl@sss.pgh.pa.us>
To: Mark Crosland <mark.crosland@gmail.com>
cc: pgsql-interfaces@postgresql.org
Subject: Re: Traversing the catalog using heap_open, systable_beginscan, ...
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Comments: In-reply-to Mark Crosland <mark.crosland@gmail.com>
 message dated "Fri, 08 Nov 2013 21:37:02 -0800"
Date: Sat, 09 Nov 2013 11:12:17 -0500
Message-ID: <31976.1384013537@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-interfaces-owner@postgresql.org

Mark Crosland <mark.crosland@gmail.com> writes:
> Hello,
> I am writing a foreign data wrapper and following the oracle example. Are
> there docs that discuss heap_open, systable_beginscan and systable_getnext?

> I currently just need to do simple stuff, like translate an Oid that is
> received via a FDW callback into the text name of a table. I see all of the
> catalog headers, pg_class.h, etc... but not seeing the pattern of which
> args to pass into ScanKeyInit to traverse the catalog correctly.

Well, you *can* look that sort of stuff up that way, but usually there's
a better way; in particular, if at all possible you should consult the
catalog caches instead.  Look into src/backend/utils/cache/lsyscache.c
for convenience routines that do this sort of thing.  The particular need
you cite above is addressed by get_rel_name(), and even if you don't see
an exact fit, the routines provide good examples for fetching fields out
of catalog entries.

Also, once you've got a Relation pointer, much of what you might want
to know about that table is available directly out of the relcache
entry.

			regards, tom lane


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