WordPress Function to Cross Taxonomy Tags Query

A cached query that outputs a get_tags() like HTML string of all tags for a given Taxonomy parameter defaulting to a Category. You can use $where_slug and $where_tax to get post tags filtered by any other taxonomy. SQL tested for WP 3.1 to WP 3.3.1.

function tags_by_other_taxonomy($where_slug,$where_tax = 'category',$bust_cache = false) {
    $cache_key = "{$where_slug}:{$where_tax}";
    $cache = get_transient('tags_by_other_taxonomy');
    $html = '';
    if( true !== $bust_cache and false !== $cache and isset($cache[$cache_key]) and !empty($cache[$cache_key]) ) {
            $html = $cache[$cache_key];
    } else {
        global $wpdb;
        $cat_id = $wpdb->get_var("SELECT tt.term_taxonomy_id FROM $wpdb->terms t INNER JOIN $wpdb->term_taxonomy tt ON t.term_id = tt.term_id WHERE t.slug = '{$where_slug}' AND tt.taxonomy = '{$where_tax}' LIMIT 1");
        if( !empty($cat_id) ) {
            $cat_posts = $wpdb->get_col("SELECT tr.object_id FROM $wpdb->term_relationships tr inner join $wpdb->posts p ON p.ID = tr.object_id WHERE term_taxonomy_id = {$cat_id} AND p.post_status = 'publish' AND p.post_type = 'post'");
            if( count($cat_posts) ) {
                $tags = $wpdb->get_results("SELECT DISTINCT t.name,t.slug FROM $wpdb->term_taxonomy tt
                                INNER JOIN $wpdb->term_relationships tr ON tt.term_taxonomy_id = tr.term_taxonomy_id
                                INNER JOIN $wpdb->terms t ON t.term_id = tt.term_id
                                WHERE tt.taxonomy = 'post_tag' and tr.object_id IN (".implode(',',$cat_posts) .')');
                $html = '<ul class="post-tags-'.$where_slug.'">';
                if( count($tags) ) {
                    foreach($tags as $tag)
                        $html .= '<li><a href="/tag/'.$tag->slug.'/" title="Posts tagged with '.$tag->name.'">'.$tag->name.'</a></li>';
                $html .= '</ul>';
                if( !is_array($cache) )
                    $cache = array();
                $cache[$cache_key] = $html;
                set_transient('sc_cross_tax', $cache, 86400);
    echo $html;

For example, get all tags for Published Posts in the news Category slug:

<?php echo tags_by_other_taxonomy('news'); ?>

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