Improve performance of trackback query in `do_all_pings()`.

Previously, the direct SQL query used to identify trackbacks in
`do_all_pings()` performed poorly, due to an unindexed query against the
`to_ping` column. We improve performance in two ways. First, we switch
to using a postmeta flag for posts that require trackbacks to be sent;
queries joining against the postmeta table that check only the `meta_key`
are generally quite fast. Second, we switch to the use of `WP_Query`,
making the query cacheable and filterable using standard methods.

Props dshanske, spacedmonkey, janw.oostendorp, mrmadhat, birgire.
Fixes #36824.

git-svn-id: https://develop.svn.wordpress.org/trunk@46178 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/comment.php

@@ -2665,15 +2665,23 @@ function do_all_pings() {
 		do_enclose( null, $enclosure->ID );
 	}
 
-	// Do Trackbacks
-	$trackbacks = $wpdb->get_col( "SELECT ID FROM $wpdb->posts WHERE to_ping <> '' AND post_status = 'publish'" );
-	if ( is_array( $trackbacks ) ) {
-		foreach ( $trackbacks as $trackback ) {
-			do_trackbacks( $trackback );
-		}
+	// Do trackbacks.
+	$trackbacks = get_posts(
+		array(
+			'post_type'        => get_post_types(),
+			'suppress_filters' => false,
+			'nopaging'         => true,
+			'meta_key'         => '_trackbackme',
+			'fields'           => 'ids',
+		)
+	);
+
+	foreach ( $trackbacks as $trackback ) {
+		delete_post_meta( $trackback, '_trackbackme' );
+		do_trackbacks( $trackback );
 	}
 
-	//Do Update Services/Generic Pings
+	// Do Update Services/Generic Pings.
 	generic_ping();
 }
 

src/wp-includes/post.php

@@ -6786,6 +6786,11 @@ function _publish_post_hook( $post_id ) {
 	}
 	add_post_meta( $post_id, '_encloseme', '1' );
 
+	$to_ping = get_to_ping( $post_id );
+	if ( ! empty( $to_ping ) ) {
+		add_post_meta( $post_id, '_trackbackme', '1' );
+	}
+
 	if ( ! wp_next_scheduled( 'do_pings' ) ) {
 		wp_schedule_single_event( time(), 'do_pings' );
 	}