要概括这一点，需要检索具有特定分类法术语B的帖子所具有的所有分类法术语A。

虽然这在几个步骤和大量的帖子循环中都不是不可能的（这确实是低效的），但我认为通过SQL提高效率是合理的。

我对此的大致看法是：

/**
 * Get all terms of $tax_to taxonomy that posts in $term_from of $tax_from have.
 *
 * @param string $tax_from  taxonomy name
 * @param string $term_from term slug
 * @param string $tax_to    taxonomy name
 *
 * @return array|WP_Error
 */
function get_intersected_terms( $tax_from, $term_from, $tax_to ) {

    global $wpdb;

    $term_from = get_term_by( \'slug\', $term_from, $tax_from );

    $query = "
    SELECT term_id FROM {$wpdb->term_taxonomy} WHERE taxonomy = \'{$tax_to}\' AND term_taxonomy_id IN (
        SELECT term_taxonomy_id FROM {$wpdb->term_relationships} WHERE object_id IN (
            SELECT object_id FROM {$wpdb->term_relationships} WHERE term_taxonomy_id = {$term_from->term_taxonomy_id}
        )
    )
    ";

    $term_ids = $wpdb->get_col( $query );

    if( empty( $term_ids) )
        return array();

    return get_terms( $tax_to, array( \'include\' => $term_ids ) );
}

// example
var_dump( get_intersected_terms( \'category\', \'cat-a\', \'post_tag\' ) );