update_post_thumbnail_cache

 *
 * @global WP_Query $wp_query WordPress Query object.
 *
 * @param WP_Query $wp_query Optional. A WP_Query instance. Defaults to the $wp_query global.
 */
function update_post_thumbnail_cache( $wp_query = null ) {
	if ( ! $wp_query ) {
		$wp_query = $GLOBALS['wp_query'];
	}

	if ( $wp_query->thumbnails_cached ) {
		return;
	}

	$thumb_ids = array();

	foreach ( $wp_query->posts as $post ) {
		$id = get_post_thumbnail_id( $post->ID );
		if ( $id ) {
			$thumb_ids[] = $id;
		}
	}