#47642

src/wp-includes/class-wp-query.php

@@ -2331,6 +2331,41 @@ public function get_posts() {
 		} 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, true ) ) {
+						$needs_ID = true;
+					} else {
+						if ( 'ID' === $_orderby ) {
+							$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, true ) ) {
+					$q['orderby'] .= ' ID'; // Add ID here, note the space
+				}
+			}
 			$orderby_array = array();
 			if ( is_array( $q['orderby'] ) ) {
 				foreach ( $q['orderby'] as $_orderby => $order ) {
@@ -2371,21 +2406,23 @@ public function get_posts() {
 		// Order search results by relevance only when another "orderby" is not specified in the query.
 		if ( ! empty( $q['s'] ) ) {
 			$search_orderby = '';
-			if ( ! empty( $q['search_orderby_title'] ) && ( empty( $q['orderby'] ) && ! $this->is_feed ) || ( isset( $q['orderby'] ) && 'relevance' === $q['orderby'] ) ) {
+			if ( ( ! empty( $q['search_orderby_title'] ) && ( empty( $q['orderby'] ) && ! $this->is_feed ) ) || ( isset( $q['orderby'] ) && 'relevance' === $q['orderby'] ) ) {
 				$search_orderby = $this->parse_search_order( $q );
 			}
 
 			if ( ! $q['suppress_filters'] ) {
 				/**
 				 * Filters the ORDER BY used when ordering search results.
-				 *
+				 *https://core.trac.wordpress.org/attachment/ticket/47642
 				 * @since 3.7.0
 				 *
 				 * @param string   $search_orderby The ORDER BY clause.
 				 * @param WP_Query $query          The current WP_Query instance.
 				 */
 				$search_orderby = apply_filters( 'posts_search_orderby', $search_orderby, $this );
 			}
+			//Add additional sort by order to break ties consistently for pagination https://core.trac.wordpress.org/ticket/47642
+			 $orderby = $orderby ? $orderby . ', ID' : ' ID';
 
 			if ( $search_orderby ) {
 				$orderby = $orderby ? $search_orderby . ', ' . $orderby : $search_orderby;

tests/phpunit/tests/query/orderby.php

@@ -0,0 +1,59 @@
+<?php
+
+/**
+ * Tests for query result ordering, specifically indeterminate ordering.
+ *
+ * @group query
+ *
+ * @covers WP_Query::posts
+ */
+class Tests_Query_Orderby extends WP_UnitTestCase {
+	function test_order_by_comment_count_does_not_cause_duplication_when_paginating() {
+		global $wpdb;
+
+		$posts = self::factory()->post->create_many( 5 );
+
+		$wpdb->update(
+			$wpdb->posts,
+			array(
+				'comment_count' => 50,
+			),
+			array(
+				'ID' => $posts[2],
+			)
+		);
+		$wpdb->update(
+			$wpdb->posts,
+			array(
+				'comment_count' => 100,
+			),
+			array(
+				'ID' => $posts[4],
+			)
+		);
+		$args = array(
+			'posts_per_page' => 1,
+			'paged'          => 1,
+			'orderby'        => 'comment_count',
+			'order'          => 'desc',
+		);
+
+		$query    = new WP_Query( $args );
+		$actual   = array();
+		$expected = array(
+			$posts[4], // 100 comments
+			$posts[2], // 50 comments
+			$posts[3], // 0 comments
+			$posts[1], // 0 comments
+			$posts[0], // 0 comments
+		);
+
+		while ( $query->have_posts() ) {
+			$actual = array_merge( $actual, wp_list_pluck( $query->posts, 'ID' ) );
+			$args['paged']++;
+			$query = new WP_Query( $args );
+		}
+
+		$this->assertSame( $expected, $actual );
+	}
+}