Re: foreign keys to foreign tables

Hello Rick,

As I understand it you are correct. Oracle/DB2/Postgres and I think the SQL
Standards to not implement constraints against tables on foreign servers.
Although it would be possible to develop the DBMS to handle such
constraints in a heterogeneous distributed environment it would be unwise
because of the poor performance and reliability of data sent over networks
so DBMSs do not implement it. You would, as you suspected, have to use
stored procedures to emulate some of the functionality of a foreign key but
definitely think twice about the performance bottlenecks you would
introduce. A more clever thing to do is use Slony, BDR, or triggers to
replicate the foreign table and create the constraint against the local
copy. In some other DBMSs the clever thing to do is create a materialized
view and constraints against the materialized view (which achieves the
same) but Postgres does not yet support such constraints against
materialized views.

*Will J. Dunn*
*willjdunn.com <http://willjdunn.com>*

On Mon, Jun 22, 2015 at 12:21 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Rick Otten <rottenwindfish(at)gmail(dot)com> writes:
> > Hello pgsql-general,
> > I'd like to set up a foreign key constraint to a foreign table from a
> local
> > table.
>
> > ie, I have a column in a local table that I'd like to ensure has a value
> in
> > the foreign table.
>
> > alter mytable
> > add column some_column_id uuid references
> myforeigntable(some_column_id)
> > ;
>
> > Unfortunately I get a "not a table" error when I try this.
>
> > ERROR: referenced relation "myforeigntable" is not a table
>
> > I'm thinking I'll have to write a function that checks for existance of
> the
> > ids in the foreign table, and then put a CHECK constraint on using that
> > function, but I thought I'd as first if there was a better way.
>
> What's going to happen when the foreign server decides to delete some rows
> from its table?
>
> regards, tom lane
>
>
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