I found out this queries by Pligg is awful.

SELECT pligg_links.link_title, pligg_links.link_title_url, COUNT( pligg_tags.tag_link_id ) AS relevance, pligg_tags.tag_link_id FROM pligg_tags, pligg_links WHERE pligg_tags.tag_words IN ( '' ) AND pligg_tags.tag_link_id = pligg_links.link_id AND pligg_links.link_status != 'discard' AND NOT pligg_links.link_id = 7323 GROUP BY pligg_tags.tag_link_id, pligg_links.link_title, pligg_links.link_title_url ORDER BY relevance DESC LIMIT 10;

This was a huge queries, and my server has ton of this, no wonder its load for mysql was high. By the way this query was not even index. Why wasn't Pligg's table has index for this type of query?

Any idea to fix this?