Re: Traversing the catalog using heap_open, systable_beginscan, ...

Mark Crosland <mark(dot)crosland(at)gmail(dot)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