Ticket #47642: patch.47642.20201030.1

Index: src/wp-includes/class-wp-query.php
===================================================================
--- src/wp-includes/class-wp-query.php  (revision 49451)
+++ src/wp-includes/class-wp-query.php  (working copy)
@@ -2325,6 +2325,34 @@
                } elseif ( 'none' === $q['orderby'] ) {
                        $orderby = '';
                } else {
+
+                       // https://core.trac.wordpress.org/ticket/47642
+                       // Database engine fix to prevent strange (duplicate or missing) results in paginated resultsets
+                       // Test for possible duplicate (non-unique, numeric or datetime) DB keys, add the unique ID
+                       $force_ID = array( 'post_author', 'post_date', 'post_date_gmt', 'post_modified', 'post_modified', 'post_modified_gmt', 'post_parent', 'comment_count' );
+                       if ( is_array( $q['orderby'] ) ) {
+                               $needs_ID = false;
+                               $has_ID = false;
+                               foreach ( $q['orderby'] as $_orderby => $order ) {
+                                       if( in_array( $_orderby, $force_ID )  ) {
+                                               $needs_ID = true;
+                                       } else {
+                                               if( $_orderby === 'ID' ) {
+                                                       $has_ID = true;
+                                                       break; // All done
+                                               }       
+                                       }
+                               }
+                               if( $needs_ID && !$has_ID ) {
+                                       $q['orderby']['ID'] = 'ASC';
+                               }
+                       } else {
+                               // Not an array, test as string
+                               if( in_array( $q['orderby'], $force_ID )  ) {
+                                       $q['orderby'] .= ' ID'; // Add ID here, note the space
+                               }
+                       }
+
                        $orderby_array = array();
                        if ( is_array( $q['orderby'] ) ) {
                                foreach ( $q['orderby'] as $_orderby => $order ) {