CREATE OR REPLACE FUNCTION my_rank (tsvector, tsquery) RETURNS float AS $$
my ($string, $query) = @_;

# Parse tsvector. Use only words, not positional information
my @vector = map /'(\w+)'/, split / /, $string;

# Parse tsquery, assuming it to be ORed only
$query =~ s/[()|'']/ /g;
$query =~ s/^\s+(.*?)\s+$/$1/;
my @query = split /\s+/, $query;

# Number of query terms in the vector
my $qlength = scalar(@query);
# Number of matched terms
my $qfound = scalar(map {my $q=$_; grep {/$q/} @vector} @query);

# The more matched terms - the better
my $rank_more = $qfound/$qlength;
# The less matched terms - the better
my $rank_less = ($qlength - $qfound + 1)/$qlength;
# Middle way - prefer both single and all matches
#my $rank_mid = 1 - $qfound*($qlength - $qfound + 1)/$qlength**2;
my $rank_mid = ($rank_more > $rank_less ? $rank_more : $rank_less);

return $rank_mid;
$$ LANGUAGE plperl;

SELECT title, my_rank(fts, q) rank FROM apod, to_tsquery('english','sombrero|supernovae|milky|way|meteor') q WHERE fts @@ q ORDER BY rank DESC LIMIT 100;