Index: src/wp-includes/ms-functions.php
===================================================================
--- src/wp-includes/ms-functions.php	(revision 27142)
+++ src/wp-includes/ms-functions.php	(working copy)
@@ -275,11 +275,18 @@
 
 	if ( $reassign != '' ) {
 		$reassign = (int) $reassign;
-		$wpdb->query( $wpdb->prepare("UPDATE $wpdb->posts SET post_author = %d WHERE post_author = %d", $reassign, $user_id) );
-		$wpdb->query( $wpdb->prepare("UPDATE $wpdb->links SET link_owner = %d WHERE link_owner = %d", $reassign, $user_id) );
+		$post_ids = $wpdb->get_col( $wpdb->prepare( "SELECT ID FROM $wpdb->posts WHERE post_author = %d", $user_id ) );
+		$link_ids = $wpdb->get_col( $wpdb->prepare( "SELECT ID FROM $wpdb->links WHERE link_owner = %d", $user_id ) );
 
-		$post_ids = $wpdb->get_col( $wpdb->prepare( "SELECT ID FROM $wpdb->posts WHERE post_author = %d", $reassign ) );
-		array_map( 'clean_post_cache', $post_ids ); 
+		if ( ! empty( $post_ids ) ) {
+			$wpdb->query( $wpdb->prepare( "UPDATE $wpdb->posts SET post_author = %d WHERE ID IN (" . implode( ',', $post_ids ) . ")", $reassign ) );
+			array_walk( $post_ids, 'clean_post_cache' );
+		}
+
+		if ( ! empty( $link_ids ) ) {
+			$wpdb->query( $wpdb->prepare( "UPDATE $wpdb->links SET link_owner = %d WHERE ID IN (" . implode( ',', $link_ids ) . ")", $reassign ) );
+			array_walk( $link_ids, 'clean_bookmark_cache' );
+		}
 	}
 
 	restore_current_blog();