_bbp_has_replies_where( string $where = '',  $query = false )

function _bbp_has_replies_where( $where = '', $query = false ) {

	/** Bail ******************************************************************/

	// Bail if the sky is falling
	if ( empty( $where ) || empty( $query ) ) {
		return $where;
	}

	// Bail if no post_parent to replace
	if ( ! is_numeric( $query->get( 'post_parent' ) ) ) {
		return $where;
	}

	// Bail if not a topic and reply query
	if ( array( bbp_get_topic_post_type(), bbp_get_reply_post_type() ) !== $query->get( 'post_type' ) ) {
		return $where;
	}

	// Bail if including or excluding specific post ID's
	if ( $query->get( 'post__not_in' ) || $query->get( 'post__in' ) ) {
		return $where;
	}

	/** Proceed ***************************************************************/

	global $wpdb;

	// Table name for posts
	$table_name = $wpdb->prefix . 'posts';

	// Get the topic ID from the post_parent, set in bbp_has_replies()
	$topic_id   = bbp_get_topic_id( $query->get( 'post_parent' ) );

	// The texts to search for
	$search     = array(
		"FROM {$table_name} " ,
		"WHERE 1=1  AND {$table_name}.post_parent = {$topic_id}"
	);

	// The texts to replace them with
	$replace     = array(
		$search[0] . "FORCE INDEX (PRIMARY, post_parent) " ,
		"WHERE 1=1 AND ({$table_name}.ID = {$topic_id} OR {$table_name}.post_parent = {$topic_id})"
	);

	// Try to replace the search text with the replacement
	$new_where = str_replace( $search, $replace, $where );
	if ( ! empty( $new_where ) ) {
		$where = $new_where;
	}

	return $where;
}