use warnings;
use strict;
use Data::Dump;
use DBI;

# set up a dummy database
my $dbh = DBI->connect("dbi:SQLite::memory:", undef, undef,
	{ RaiseError=>1 });
$dbh->do(<<'END');
CREATE TABLE messages (
	msgID INTEGER,
	threadID INTEGER
)
END
my $sth_i = $dbh->prepare(
	'INSERT INTO messages (msgID, threadID) VALUES (?,?)');
$sth_i->execute($_,$_>>4) for 0..127;

# ask the DB to give us per-thread information
dd $dbh->selectall_arrayref(
	'SELECT threadID, COUNT(*) FROM messages GROUP BY threadID');
# => [ [0, 16], [1, 16], [2, 16], [3, 16],
#      [4, 16], [5, 16], [6, 16], [7, 16], ]

# loop through rows and count threads ourselves
my $sth_s = $dbh->prepare(
	'SELECT msgID, threadID from messages');
$sth_s->execute;
my %threadIDs;
while ( my $row = $sth_s->fetchrow_hashref ) {
	$threadIDs{ $row->{threadID} }++;
}
dd \%threadIDs;
# => { "0" => 16, "1" => 16, "2" => 16, "3" => 16,
#      "4" => 16, "5" => 16, "6" => 16, "7" => 16 }