dblink_get_pkey — returns the positions and field names of a relation's primary key fields
dblink_get_pkey(text relname) returns setof dblink_pkey_results
    dblink_get_pkey provides information about the primary
    key of a relation in the local database.  This is sometimes useful
    in generating queries to be sent to remote databases.
   
relname       Name of a local relation, for example foo or
       myschema.mytab.  Include double quotes if the
       name is mixed-case or contains special characters, for
       example "FooBar"; without quotes, the string
       will be folded to lower case.
      
Returns one row for each primary key field, or no rows if the relation has no primary key. The result row type is defined as
CREATE TYPE dblink_pkey_results AS (position int, colname text);
    The position column simply runs from 1 to N;
    it is the number of the field within the primary key, not the number
    within the table's columns.
   
CREATE TABLE foobar (
    f1 int,
    f2 int,
    f3 int,
    PRIMARY KEY (f1, f2, f3)
);
CREATE TABLE
SELECT * FROM dblink_get_pkey('foobar');
 position | colname
----------+---------
        1 | f1
        2 | f2
        3 | f3
(3 rows)