Improper testing of cache retrievals causes wasted queries.

[andy]

wp_cache_get() can retrieve values such as the empty array which is considered false by the ! test. The following function is incorrect because it will always ignore the cache and repeat the get_col and wp_cache_add when the cache contains the empty array. This occurs on sites which have no pages.

function get_all_page_ids() {
        global $wpdb;

        if ( ! $page_ids = wp_cache_get('all_page_ids', 'posts') ) {
                $page_ids = $wpdb->get_col("SELECT ID FROM $wpdb->posts WHERE post_type = 'page'");
                wp_cache_add('all_page_ids', $page_ids, 'posts');
        }

        return $page_ids;
}

The cache check should be written with an understanding of the return type of $wpdb->get_col() (array). Here it is with the check fixed:

function get_all_page_ids() {
	global $wpdb;

	$page_ids = wp_cache_get('all_page_ids', 'posts');
	if ( ! is_array( $page_ids ) ) {
		$page_ids = $wpdb->get_col("SELECT ID FROM $wpdb->posts WHERE post_type = 'page'");
		wp_cache_add('all_page_ids', $page_ids, 'posts');
	}

	return $page_ids;
}

Patch 1 fixes this occurrence. Please keep this ticket open until all similar occurrences are found and fixed.

fix get_all_page_ids()

I always thought that (bool) array() evaluates to true, but that's obviously not the case:

​http://ch2.php.net/manual/en/language.types.boolean.php

In [20216]:

Don't generate a query when the all_page_ids cache is an empty array. An empty array is a valid cache value when no pages are present. Avoids a useless query for sites with no pages. Props andy. see #20236