sub SQL_FUNCTION_POSITION {
        my($sth,$rowhash,@params) = @_;
        return index($params[1],$params[0]) +1;
    }
    # e.g. SELECT col1 FROM tbl WHERE POSITION('foo',col2) < 3;

##</code><code>##

sub SOUNDEX {
  my($self,$sth,$rowhash,@params)=@_;
  use Text::Soundex 'soundex';
  my $s1 = soundex($params[0]) or return 0;
  my $s2 = soundex($params[1]) or return 0;
  return $s1 eq $s2;
}

##</code><code>##

  use DBI;
  my $dbh = DBI->connect('dbi:File(RaiseError=1):');
  $dbh->do("CREATE FUNCTION $_ EXTERNAL") for qw(Prof Class);
  sub Prof  {[ [qw(pid pname)],[qw(1 Sue )],[qw(2 Bob)],[qw(3 Tom )] ]}
  sub Class {[ [qw(pid cname)],[qw(1 Chem)],[qw(2 Bio)],[qw(2 Math)] ]}
  $sth = $dbh->prepare("SELECT * FROM Prof NATURAL JOIN Class");
  $sth->execute;
  $sth->dump_results;

##</code><code>##

$sth = $dbh->prepare("SELECT * FROM _1(?) AS x NATURAL JOIN _2(?) AS y");
$sth->execute( $pg_sth, $mysql_sth);