Get post ID using custom field value

I am using a function to exclude certain posts from the main site feed using post IDs. It’s working by directly inputting the post ID in the function, but when I tried to do the same by using custom fields, it is not working.

This is the function:

function exp_post($query) {
	global $wpdb;
	$pid = "SELECT * FROM $wpdb->postmeta WHERE meta_key = 'excludefeed'";
	$posts = $wpdb->get_results($pid);
	foreach( $posts as $post ) {
		$exclude[] = $post->ID;
	}
    if ( $query->is_feed) {
        $query->set('post__not_in', $exclude );
    }
    return $query;
}
add_filter( 'pre_get_posts', 'exp_post' );

What seems to be the problem?