Exclude WC categories from widget that only contain out of stock products Raw

wpse74896.php

<?php

// Inspired by this WPSE question: http://wordpress.stackexchange.com/questions/74896/woocommerce-product-category-widget-hide-categories-that-have-no-products-in-s

add_filter( 'woocommerce_product_categories_widget_args', 'wpse_74896' );
function wpse_74896( $args ) {
    global $wpdb;
    if ( ! empty( $args['current_category'] ) ) {
        $current = $args['current_category'];
        $current_clause = "( term_id = '{$current}' OR parent = '{$current}' ) AND ";
    } else {
        $current_clause = "";
    }
    $query = "SELECT term_id
            FROM $wpdb->term_taxonomy
            WHERE {$current_clause} term_taxonomy_id IN (
                SELECT DISTINCT term_taxonomy_id
                FROM $wpdb->term_relationships
                WHERE object_id IN (
                    SELECT DISTINCT post_id
                    FROM $wpdb->postmeta
                    WHERE post_id IN (
                        SELECT id FROM $wpdb->posts WHERE post_type LIKE 'product' AND post_status LIKE 'publish'
                    )
                    AND post_id NOT IN (
                        SELECT DISTINCT post_id
                        FROM $wpdb->postmeta
                        WHERE (
                            ( meta_key = '_stock_status' AND meta_value = 'outofstock' )
                            OR ( meta_key = '_stock' AND meta_value != '' AND meta_value < 1 )
                        )
                    )
                )
            )
            AND taxonomy LIKE 'product_cat'";
    $result = $wpdb->get_col( $query );
    if ( is_array( $result ) && ! empty( $result ) ) {
        $args['include'] = $result;
    }
    return $args;
}

Ideas for potential improvement, in no particular order:

• optimize SQL query
• store results in a transient (and invalidate transient whenever a product's inventory level is changed?)