dblink_build_sql_update — builds an UPDATE statement using a local tuple, replacing the primary key field values with alternative supplied values
dblink_build_sql_update(text relname,
                        int2vector primary_key_attnums,
                        integer num_primary_key_atts,
                        text[] src_pk_att_vals_array,
                        text[] tgt_pk_att_vals_array) returns text
    dblink_build_sql_update can be useful in doing selective
    replication of a local table to a remote database.  It selects a row
    from the local table based on primary key, and then builds a SQL
    UPDATE command that will duplicate that row, but with
    the primary key values replaced by the values in the last argument.
    (To make an exact copy of the row, just specify the same values for
    the last two arguments.)  The UPDATE command always assigns
    all fields of the row — the main difference between this and
    dblink_build_sql_insert is that it's assumed that
    the target row already exists in the remote table.
   
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.
      
primary_key_attnums
       Attribute numbers (1-based) of the primary key fields,
       for example 1 2.
      
num_primary_key_attsThe number of primary key fields.
src_pk_att_vals_arrayValues of the primary key fields to be used to look up the local tuple. Each field is represented in text form. An error is thrown if there is no local row with these primary key values.
tgt_pk_att_vals_array
       Values of the primary key fields to be placed in the resulting
       UPDATE command.  Each field is represented in text form.
      
Returns the requested SQL statement as text.
    As of PostgreSQL 9.0, the attribute numbers in
    primary_key_attnums are interpreted as logical
    column numbers, corresponding to the column's position in
    SELECT * FROM relname.  Previous versions interpreted the
    numbers as physical column positions.  There is a difference if any
    column(s) to the left of the indicated column have been dropped during
    the lifetime of the table.
   
SELECT dblink_build_sql_update('foo', '1 2', 2, '{"1", "a"}', '{"1", "b"}');
                   dblink_build_sql_update
-------------------------------------------------------------
 UPDATE foo SET f1='1',f2='b',f3='1' WHERE f1='1' AND f2='b'
(1 row)