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SQL script replacing column's data
BEGIN;

    CREATE TEMPORARY TABLE tmp_table (id bigint PRIMARY KEY,
        registrationnumber character varying(255));
    INSERT INTO tmp_table
    select id,registrationnumber from tableB;

    for d in tmp_table loop
        update TABLEA set registrationnumber=d.id where
            registrationnumber=d.registrationnumber;
        return next d;
    end loop;

END;

UPDATE tablea
  SET registrationnumber = tableb.id
  FROM tableb
 WHERE tablea.registrationnumber = tableb.registrationnumber;

select * from tablea;