Re: FDW question - how to identify columns to populate in response?

Bear Giles <bgiles@coyotesong.com> writes:
> Hi, I'm working on a FDW for the unix/linux user database - think
> /etc/passwd and /etc/group although I'm actually using system calls that
> could be quietly redirected to LDAP or other backends. It's easy to create
> the FDW and a table associated with it, something like

> CREATE TABLE passwd (
>    name text,
>    passwd text,
>    uid int,
>    ...

> The problem is the user could decide to reorder or remove columns so I
> can't make the assumption that values[0] is always going to be the username.

> I have a solution that requires looking at the rel, extracting the atts,
> and then doing a loop where I check the attname against all possible values
> for each column. Anything that doesn't match is set to null. This isn't too
> bad here but it would be a pain if there are many columns.

> Is there a cleaner way?

The only thing that comes to mind is that you could probably amortize the
figure-out-the-mapping overhead over multiple tuples.  There's surely no
reason to do it more often than once per query; and if that's not
good-enough performance you could think about caching it longer, with some
sort of invalidation logic.

Take a look at src/include/access/tupconvert.h and
src/backend/access/common/tupconvert.c for inspiration, or maybe even code
you can use directly.
        regards, tom lane