_find_post_by_old_date

find_post_by_old_date: 这个函数用来根据旧的日期查找一个文章: 这个函数是WordPress内部使用的，用来寻找已经被移到新日期的文章。

查找重定向一个旧日期的文章ID。

function _find_post_by_old_date( $post_type ) {
	global $wpdb;

	$date_query = '';
	if ( get_query_var( 'year' ) ) {
		$date_query .= $wpdb->prepare( ' AND YEAR(pm_date.meta_value) = %d', get_query_var( 'year' ) );
	}
	if ( get_query_var( 'monthnum' ) ) {
		$date_query .= $wpdb->prepare( ' AND MONTH(pm_date.meta_value) = %d', get_query_var( 'monthnum' ) );
	}
	if ( get_query_var( 'day' ) ) {
		$date_query .= $wpdb->prepare( ' AND DAYOFMONTH(pm_date.meta_value) = %d', get_query_var( 'day' ) );
	}

	$id = 0;
	if ( $date_query ) {
		$query        = $wpdb->prepare( "SELECT post_id FROM $wpdb->postmeta AS pm_date, $wpdb->posts WHERE ID = post_id AND post_type = %s AND meta_key = '_wp_old_date' AND post_name = %s" . $date_query, $post_type, get_query_var( 'name' ) );
		$key          = md5( $query );
		$last_changed = wp_cache_get_last_changed( 'posts' );
		$cache_key    = "find_post_by_old_date:$key:$last_changed";
		$cache        = wp_cache_get( $cache_key, 'posts' );
		if ( false !== $cache ) {
			$id = $cache;
		} else {
			$id = (int) $wpdb->get_var( $query );
			if ( ! $id ) {
				// Check to see if an old slug matches the old date.
				$id = (int) $wpdb->get_var( $wpdb->prepare( "SELECT ID FROM $wpdb->posts, $wpdb->postmeta AS pm_slug, $wpdb->postmeta AS pm_date WHERE ID = pm_slug.post_id AND ID = pm_date.post_id AND post_type = %s AND pm_slug.meta_key = '_wp_old_slug' AND pm_slug.meta_value = %s AND pm_date.meta_key = '_wp_old_date'" . $date_query, $post_type, get_query_var( 'name' ) ) );
			}
			wp_cache_set( $cache_key, $id, 'posts' );
		}
	}

	return $id;
}