发现blog的一个bug

（如果传递了blog_id，则后面还有where a.blog_id=xx and c.blog_id=yy的条件）:

SELECT c.id AS id, c.name AS name, c.url AS url, c.blog_id AS blog_id, c.last_modification AS last_modification, c.in_main_page AS in_main_page, c.parent_id AS parent_id, c.properties AS properties, c.description AS description, c.mangled_name AS mangled_name, IF(a.id IS NULL, 0, COUNT(*)) AS num_articles, a.date AS last_update FROM pg_articles_categories c LEFT JOIN pg_article_categories_link l ON c.id=l.category_id LEFT JOIN pg_articles a ON a.id = l.article_id WHERE a.id = 38064 GROUP BY c.id;

这条sql运行非常慢，以致当系统出现这条sql的时候，常常会耗尽资源而崩溃，之前blog的多次down机事件就是这样造成的。

这条sql语句在连接条件上都有索引，为什么耗资源还需要研究，相信应可以找出解决办法的