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    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;

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

  use DBI;
  my $dbh = DBI->connect('dbi:File(RaiseError=1):');
...
  $sth = $dbh->prepare("SELECT * FROM Prof NATURAL JOIN Class");
  $sth->execute;
  $sth->dump_results;

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