Refactor WP_Query ordering logic to implement a blacklist approach for deterministic ordering. Updated comments for clarity and introduced a new test for metadata ordering to ensure no duplicates across paginated results.

Author: ramonjd

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

@@ -2519,24 +2519,14 @@ public function get_posts() {
 			 * Ensure deterministic ordering to prevent duplicate records across pages.
 			 * When multiple posts have the same value for a field, add ID as secondary sort to guarantee consistent ordering.
 			 * Note: this is to circumvent a bug that is currently being tracked in https://core.trac.wordpress.org/ticket/44349.
+			 *
+			 * Use a blacklist approach: add ID as tie-breaker for all orderby fields except those that are
+			 * already deterministic (ID itself, random ordering, or search relevance).
 			 */
-			$fields_requiring_deterministic_orderby = array(
-				'post_name',
-				'post_author',
-				'post_date',
-				'post_title',
-				'post_modified',
-				'post_parent',
-				'post_type',
-				'name',
-				'author',
-				'date',
-				'title',
-				'modified',
-				'parent',
-				'type',
-				'menu_order',
-				'comment_count',
+			$fields_excluding_deterministic_orderby = array(
+				'ID',
+				'rand',
+				'relevance',
 			);
 
 			$orderby_array               = array();
@@ -2555,10 +2545,10 @@ public function get_posts() {
 
 					$orderby_array[] = $parsed . ' ' . $this->parse_order( $order );
 
-					// Check if this field needs deterministic ordering
-					if ( in_array( $_orderby, $fields_requiring_deterministic_orderby, true ) ) {
+					// Check if this field should have deterministic ordering (not in blacklist).
+					if ( ! in_array( $_orderby, $fields_excluding_deterministic_orderby, true ) ) {
 						$needs_deterministic_orderby = true;
-						// Use the order from the array for ID tie-breaker
+						// Use the order from the array for ID tie-breaker.
 						$id_tie_breaker_order = $this->parse_order( $order );
 					} elseif ( 'ID' === $_orderby ) {
 						$has_id_orderby = true;
@@ -2577,21 +2567,21 @@ public function get_posts() {
 
 					$orderby_array[] = $parsed . ' ' . $query_vars['order'];
 
-					// Check if this field needs deterministic ordering
-					if ( in_array( $orderby, $fields_requiring_deterministic_orderby, true ) ) {
+					// Check if this field should have deterministic ordering (not in blacklist).
+					if ( ! in_array( $orderby, $fields_excluding_deterministic_orderby, true ) ) {
 						$needs_deterministic_orderby = true;
 					} elseif ( 'ID' === $orderby ) {
 						$has_id_orderby = true;
 					}
 				}
 			}
 
-			// Add ID as tie-breaker if needed and not already present
+			// Add ID as tie-breaker if needed and not already present.
 			if ( $needs_deterministic_orderby && ! $has_id_orderby ) {
 				$orderby_array[] = "{$wpdb->posts}.ID " . $id_tie_breaker_order;
 			}
 
-			// Build the final orderby string
+			// Build the final orderby string.
 			if ( empty( $orderby_array ) ) {
 				$orderby = "{$wpdb->posts}.post_date " . $query_vars['order'] . ', ' . "{$wpdb->posts}.ID " . $query_vars['order'];
 			} else {

tests/phpunit/tests/query/deterministicOrdering.php

@@ -427,4 +427,74 @@ public function test_deterministic_ordering_with_menu_order() {
 
 		$this->assertEquals( $page1_ids, $page1_repeat_ids, 'Same query should return same results when ordering by menu_order' );
 	}
+
+	/**
+	 * Test that deterministic ordering works with metadata ordering.
+	 *
+	 * @ticket xxxxx
+	 */
+	public function test_deterministic_ordering_with_metadata() {
+		$post_ids = array();
+
+		// Create posts with identical meta values to trigger the bug
+		$identical_meta_value = 'same_price';
+		for ( $i = 1; $i <= 20; $i++ ) {
+			$post_id = self::factory()->post->create(
+				array(
+					'post_type'  => 'wptests_time_ident',
+					'post_title' => "Post $i",
+				)
+			);
+			add_post_meta( $post_id, 'price', $identical_meta_value );
+			$post_ids[] = $post_id;
+		}
+
+		// Get first page ordering by metadata
+		$query1 = new WP_Query(
+			array(
+				'post_type'      => 'wptests_time_ident',
+				'post__in'       => $post_ids,
+				'meta_query'     => array(
+					'price_key' => array(
+						'key'     => 'price',
+						'compare' => 'EXISTS',
+					),
+				),
+				'orderby'        => 'price_key',
+				'order'          => 'ASC',
+				'posts_per_page' => 10,
+				'paged'          => 1,
+			)
+		);
+
+		// Get second page ordering by metadata
+		$query2 = new WP_Query(
+			array(
+				'post_type'      => 'wptests_time_ident',
+				'post__in'       => $post_ids,
+				'meta_query'     => array(
+					'price_key' => array(
+						'key'     => 'price',
+						'compare' => 'EXISTS',
+					),
+				),
+				'orderby'        => 'price_key',
+				'order'          => 'ASC',
+				'posts_per_page' => 10,
+				'paged'          => 2,
+			)
+		);
+
+		$page1_ids = wp_list_pluck( $query1->posts, 'ID' );
+		$page2_ids = wp_list_pluck( $query2->posts, 'ID' );
+
+		// Verify no overlap between pages (no duplicates)
+		$overlap = array_intersect( $page1_ids, $page2_ids );
+		$this->assertEmpty( $overlap, 'Pages should not contain duplicate posts when ordering by metadata' );
+
+		// Verify total count is correct
+		$this->assertEquals( 20, $query1->found_posts, 'Total posts should be 20' );
+		$this->assertEquals( 10, count( $page1_ids ), 'First page should have 10 posts' );
+		$this->assertEquals( 10, count( $page2_ids ), 'Second page should have 10 posts' );
+	}
 }