refactor(media): Refactor month retrieval and add new function for easier caching

src/wp-includes/media.php

@@ -4869,10 +4869,11 @@ function wp_enqueue_media( $args = array() ) {
 	/**
 	 * Allows overriding the list of months displayed in the media library.
 	 *
-	 * By default (if this filter does not return an array), a query will be
-	 * run to determine the months that have media items.  This query can be
-	 * expensive for large media libraries, so it may be desirable for sites to
-	 * override this behavior.
+	 * By default, if this filter does not return an array,
+	 * {@see wp_get_media_library_months_with_files()} will run a query to determine
+	 * the months that have media items. The result is stored in a transient
+	 * and automatically invalidated when attachments are created, updated,
+	 * or deleted.
 	 *
 	 * @since 4.7.4
 	 *
@@ -4883,16 +4884,13 @@ function wp_enqueue_media( $args = array() ) {
 	 */
 	$months = apply_filters( 'media_library_months_with_files', null );
 	if ( ! is_array( $months ) ) {
-		$months = $wpdb->get_results(
-			$wpdb->prepare(
-				"SELECT DISTINCT YEAR( post_date ) AS year, MONTH( post_date ) AS month
-				FROM $wpdb->posts
-				WHERE post_type = %s
-				ORDER BY post_date DESC",
-				'attachment'
-			)
-		);
+		$months = get_transient( 'media_library_months_with_files' );
+		if ( false === $months ) {
+			$months = wp_get_media_library_months_with_files();
+			set_transient( 'media_library_months_with_files', $months );
+		}
 	}
+
 	foreach ( $months as $month_year ) {
 		$month_year->text = sprintf(
 			/* translators: 1: Month, 2: Year. */
@@ -5152,6 +5150,28 @@ function wp_enqueue_media( $args = array() ) {
 	do_action( 'wp_enqueue_media' );
 }
 
+/**
+ * Retrieves the months that have media items.
+ *
+ * @since tbd
+ *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
+ * @return array<int, object{year: string, month: string}> Array of objects with `month` and `year` properties.
+ */
+function wp_get_media_library_months_with_files(): array {
+	global $wpdb;
+	return $wpdb->get_results(
+		$wpdb->prepare(
+			"SELECT DISTINCT YEAR( post_date ) AS year, MONTH( post_date ) AS month
+				FROM $wpdb->posts
+				WHERE post_type = %s
+				ORDER BY post_date DESC",
+			'attachment'
+		)
+	);
+}
+
 /**
  * Retrieves media attached to the passed post.
  *

src/wp-includes/post.php

@@ -7791,6 +7791,10 @@ function clean_post_cache( $post ) {
 		do_action( 'clean_page_cache', $post->ID );
 	}
 
+	if ( 'attachment' === $post->post_type ) {
+		delete_transient( 'media_library_months_with_files' );
+	}
+
 	wp_cache_set_posts_last_changed();
 }