Ticket #42256: 42256.2.diff

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

@@ -1917 +1917 @@
                 } elseif ( $q['post__in'] ) {
                         $post__in = implode(',', array_map( 'absint', $q['post__in'] ));
                         $where .= " AND {$wpdb->posts}.ID IN ($post__in)";
+
+                        // Don't count found rows when a specific range is requested.
+                        if ( isset( $q['posts_per_page'] ) && count( $q['post__in'] ) === (int) $q['posts_per_page'] ) {
+                                $q['no_found_rows'] = true;
+                        }
                 } elseif ( $q['post__not_in'] ) {
                         $post__not_in = implode(',',  array_map( 'absint', $q['post__not_in'] ));
                         $where .= " AND {$wpdb->posts}.ID NOT IN ($post__not_in)";

tests/phpunit/tests/query/noFoundRows.php

@@ -86 +86 @@
                 $this->assertNotContains( 'SQL_CALC_FOUND_ROWS', $q->request );
                 $this->assertSame( 1, $q->found_posts );
         }
+
+        /**
+         * @ticket 42256
+         */
+        public function test_no_found_rows_true_when_post__in_count_matches_posts_per_page() {
+                $post_ids = $this->factory->post->create_many( 3 );
+
+                $q = new WP_Query( array(
+                        'post__in' => $post_ids,
+                        'posts_per_page' => count( $post_ids ),
+                ) );
+
+                $this->assertNotContains( 'SQL_CALC_FOUND_ROWS', $q->request );
+        }
 }