what's the output of this query?

I want to get all the data with the meta key of “project_id” from post meta table. I wrote a query, it should work in thoery, but I have no way to see what I get, and inject this query to my plugin doesn’t generate anything. Please take a look at this query, and help me understand if it is correct, what’s the formate of the outcome, How can I echo it to see what I get:

function mp_all_ids( ){
global $bp, $wpdb, $post;

$query = <<<QUERY
SELECT      project_id.post_id
FROM        $wpdb->postmeta project_id
LEFT JOIN  $wpdb->postmeta height
            on height.post_id = project_id.post_id
            and height.meta_key = 'height'
LEFT JOIN  $wpdb->postmeta width
            on width.post_id = project_id.post_id
            and width.meta_key = 'width'
WHERE       project_id.meta_key = 'project_id'
ORDER BY    project_id.meta_value+(0) ASC
QUERY;
$data = $wpdb->get_results( $query );

return apply_filters( 'mp_ids', $data );
}