Prime caches when retrieving posts in the sitemaps

inc/sitemaps/class-post-type-sitemap-provider.php

@@ -566,6 +566,17 @@ protected function get_posts( $post_type, $count, $offset ) {
 			$post_ids[] = $sanitized_post->ID;
 		}
 
+		/**
+		 * Filter to disable priming the post and term caches for the sitemap.
+		 *
+		 * @since 28.0
+		 *
+		 * @param bool $disable_priming_post_caches Whether to disable priming the post and term caches. Defaults to false.
+		 */
+		if ( ! apply_filters( 'wpseo_disable_priming_post_caches_sitemap', false ) ) {
+			// Warm the post and term caches in bulk, so permalink and image building doesn't query per post.
+			_prime_post_caches( $post_ids, true, false );
+		}
 		update_meta_cache( 'post', $post_ids );
 
 		return $posts;

tests/WP/Sitemaps/Post_Type_Sitemap_Provider_Test.php

@@ -404,6 +404,83 @@ public function test_password_protected_post_parent_attachment() {
 		$this->assertCount( 0, self::$class_instance->get_sitemap_links( 'attachment', 100, 0 ) );
 	}
 
+	/**
+	 * Tests that generating sitemap links runs a constant number of queries, regardless of the post count.
+	 *
+	 * @covers ::get_sitemap_links
+	 *
+	 * @return void
+	 */
+	public function test_get_sitemap_links_query_count_does_not_grow_with_post_count() {
+		// A %category% permalink structure makes get_permalink() look up each post's terms.
+		$this->set_permalink_structure( '/%category%/%postname%/' );
+
+		$this->create_posts_with_own_category( 3 );
+		$queries_for_three_posts = $this->count_sitemap_links_queries();
+
+		$this->create_posts_with_own_category( 7 );
+		$queries_for_ten_posts = $this->count_sitemap_links_queries();
+
+		$this->assertSame(
+			$queries_for_three_posts,
+			$queries_for_ten_posts,
+			'Generating sitemap links should not run more queries when there are more posts',
+		);
+	}
+
+	/**
+	 * Tests that the cache priming can be disabled through the wpseo_disable_priming_post_caches_sitemap filter.
+	 *
+	 * @covers ::get_sitemap_links
+	 *
+	 * @return void
+	 */
+	public function test_priming_post_caches_can_be_disabled_via_filter() {
+		// A %category% permalink structure makes get_permalink() look up each post's terms.
+		$this->set_permalink_structure( '/%category%/%postname%/' );
+
+		$this->create_posts_with_own_category( 3 );
+		$queries_with_priming = $this->count_sitemap_links_queries();
+
+		\add_filter( 'wpseo_disable_priming_post_caches_sitemap', '__return_true' );
+		$queries_without_priming = $this->count_sitemap_links_queries();
+		\remove_filter( 'wpseo_disable_priming_post_caches_sitemap', '__return_true' );
+
+		$this->assertGreaterThan(
+			$queries_with_priming,
+			$queries_without_priming,
+			'Disabling the cache priming through the filter should make per-post queries return',
+		);
+	}
+
+	/**
+	 * Creates posts that each have their own category.
+	 *
+	 * @param int $count The number of posts to create.
+	 *
+	 * @return void
+	 */
+	private function create_posts_with_own_category( $count ) {
+		for ( $i = 0; $i < $count; $i++ ) {
+			$category_id = $this->factory->category->create();
+			$this->factory->post->create( [ 'post_category' => [ $category_id ] ] );
+		}
+	}
+
+	/**
+	 * Counts the number of database queries run to generate the post sitemap links, starting from a cold cache.
+	 *
+	 * @return int The number of queries.
+	 */
+	private function count_sitemap_links_queries() {
+		\wp_cache_flush();
+
+		$queries_before = \get_num_queries();
+		self::$class_instance->get_sitemap_links( 'post', 100, 1 );
+
+		return ( \get_num_queries() - $queries_before );
+	}
+
 	/**
 	 * Filter to exclude desired posts from the sitemap.
 	 *