How to UPDATE faster with GROUP BY?
UPDATE articles a JOIN ( 
  SELECT a2.ID,COUNT(*) AS C 
  FROM articles a2 JOIN tag_map b2 ON a2.id=b2.id 
  WHERE a2.tags IS NULL
  GROUP BY a2.ID 
) b ON a.ID=b.ID  
SET a.Tags=b.C 
WHERE a.Tags IS NULL;