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

Thanks, that helps. I can get table and column info from the catalog now.

What is the recommended way (structs to access via one of the scan/modify
callbacks?) to retrieve the column, operation and value, "col", "=" and
"val" in this example. And is the same method used for both queries and
updates?
select * from table where col=val

A lot of the FDW examples seem to use table options, which isn't quite what
we need. We are going for more of a mapping between tables in different DBs.

Thanks,

On Sat, Nov 9, 2013 at 8:12 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> 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
>