our $sel1=qq[
    select count(*)
];
our $sel2=qq[
    select count(A.wsf_id)
];
our $str=qq[
    from wsf_price_stats A,wsf_price_stats B
    where A.wsf_id = 30472036
    and B.wsf_id   = 30472008
    and A.zp_id    = B.zp_id
];

use Benchmark qw(cmpthese);
$|++;
cmpthese 1000, {
    'count(*)'      => q[my $r=$dbh->selectrow_arrayref($sel1.$str) or die "$str:".$dbh->errstr();],
    'count(wsf_id)' => q[my $r=$dbh->selectrow_arrayref($sel2.$str) or die "$str:".$dbh->errstr();],
    'perl'          => q[get_isect()],
};

sub get_isect {
    my (@col_1,@col_2,%isect,%union);

    my $sql = "SELECT zp_id FROM wsf_price_stats WHERE wsf_id = 30472036";
    my $sth = $dbh->prepare($sql) or die("Could not prepare!" . $dbh->errstr);
    $sth->execute() or die("Could not execute 1!" . $dbh->errstr);
    while (my $data = $sth->fetchrow_array()) {
        push(@col_1, $data);
    }
    $sql = "SELECT zp_id FROM wsf_price_stats WHERE wsf_id = 30472008";
    $sth = $dbh->prepare($sql) or die("Could not prepare!" . $dbh->errstr);
    $sth->execute() or die("Could not execute 2!" . $dbh->errstr);
    while (my $data = $sth->fetchrow_array()) {
        push(@col_2, $data);
    }

    foreach my $e (@col_2) { $union{$e} = 1 }
    foreach my $e (@col_1) {
        if ( $union{$e} ) { $isect{$e} = 1 }
    }
    [scalar keys %isect]
}


__END__

Benchmark: timing 1000 iterations of count(*), count(wsf_id), perl...
  count(*): 17 wallclock secs ( 0.53 usr +  0.20 sys =  0.73 CPU) @ 1360.54/s (n=1000)
count(wsf_id): 16 wallclock secs ( 0.47 usr +  0.11 sys =  0.58 CPU) @ 1730.10/s (n=1000)
      perl: 403 wallclock secs ( 6.33 usr +  0.22 sys =  6.55 CPU) @ 152.77/s (n=1000)
                Rate          perl      count(*) count(wsf_id)
perl           153/s            --          -89%          -91%
count(*)      1361/s          791%            --          -21%
count(wsf_id) 1730/s         1033%           27%            --