MySQL查询太慢？

Question

所以我有一个查询：

SELECT EWRporta2_articles.*, xf_thread.*, xf_forum.*, xf_user.*, xf_post.message, 
    IF(NOT ISNULL(xf_user.user_id), xf_user.username, xf_thread.username) AS username 
FROM EWRporta2_articles 
    INNER JOIN xf_thread ON (xf_thread.thread_id = EWRporta2_articles.thread_id) 
    INNER JOIN xf_forum ON (xf_forum.node_id = xf_thread.node_id) 
    INNER JOIN xf_post ON (xf_post.post_id = xf_thread.first_post_id) 
    LEFT JOIN xf_user ON (xf_user.user_id = xf_thread.user_id) 
WHERE EWRporta2_articles.article_date < 1417987751 
    AND xf_thread.discussion_state = 'visible' 
ORDER BY EWRporta2_articles.article_date DESC 
LIMIT 0, 5 

该查询在0.0012秒执行......那好。这个查询所做的是查询文章列表，然后将它们链接到我论坛上的一个主题。

但是，我试图稍微改变查询。虽然上面的查询要求线程存在文章行。我想查找链接到特定文章或存在于特定论坛节点ID中的线索。因此，即使线程中不存在文章行，但如果它具有特定的node_id，它仍会显示出来。这是我对此的查询：

SELECT EWRporta2_articles.*, xf_thread.*, xf_forum.*, xf_user.*, xf_post.message, 
    IF(EWRporta2_articles.article_date IS NULL, xf_thread.post_date, EWRporta2_articles.article_date) AS article_date, 
    IF(NOT ISNULL(xf_user.user_id), xf_user.username, xf_thread.username) AS username 
FROM xf_thread 
    LEFT JOIN EWRporta2_articles ON (EWRporta2_articles.thread_id = xf_thread.thread_id) 
    INNER JOIN xf_forum ON (xf_forum.node_id = xf_thread.node_id) 
    INNER JOIN xf_post ON (xf_post.post_id = xf_thread.first_post_id) 
    LEFT JOIN xf_user ON (xf_user.user_id = xf_thread.user_id) 
WHERE (xf_thread.node_id IN ('66','78') OR EWRporta2_articles.article_date IS NOT NULL) 
    AND IF(EWRporta2_articles.article_date IS NULL, xf_thread.post_date, EWRporta2_articles.article_date) < 1417987751 
    AND xf_thread.discussion_state = 'visible' 
ORDER BY article_date DESC 
LIMIT 0, 5 

此查询的问题是它在0.5683秒内执行。

我能做些什么来提高性能吗？

Answer 1

首先 - 永远不要做：

​​

使用functon防止任何数据库，以便优化这样一个表达式使用索引。

把它改写成这种形式：

EWRporta2_articles.article_date IS NULL AND xf_thread.post_date < 1417987751 
OR 
EWRporta2_articles.article_date IS NOT NULL AND EWRporta2_articles.article_date < 1417987751 

---

现在使用布尔代数的法律，简化了查询的条件， 特别是在利用法律：Distirbutivity of AND over OR：http://en.wikipedia.org/wiki/Boolean_algebra
x和（y OR Z）=> X和Y或X和Z



如果您将此法应用于此条件：

WHERE 
(xf_thread.node_id IN ('66','78') OR EWRporta2_articles.article_date IS NOT NULL) 
AND 
(
    EWRporta2_articles.article_date IS NULL AND xf_thread.post_date < 1417987751 
    OR 
    EWRporta2_articles.article_date IS NOT NULL AND EWRporta2_articles.article_date < 1417987751 
) 

您将获得：

xf_thread.node_id IN ('66','78') AND EWRporta2_articles.article_date IS NULL AND xf_thread.post_date < 1417987751 
OR 
xf_thread.node_id IN ('66','78') AND EWRporta2_articles.article_date IS NOT NULL AND EWRporta2_articles.article_date < 1417987751 
OR 
EWRporta2_articles.article_date IS NOT NULL AND EWRporta2_articles.article_date IS NULL AND xf_thread.post_date < 1417987751 
OR 
EWRporta2_articles.article_date IS NOT NULL AND EWRporta2_articles.article_date IS NOT NULL AND EWRporta2_articles.article_date < 1417987751 

---

第三个条件是总是假的，所以我们可以跳过它：

EWRporta2_articles.article_date IS NOT NULL AND EWRporta2_articles.article_date IS NULL ... 

---

第二个和第四一个：

xf_thread.node_id IN ('66','78') AND EWRporta2_articles.article_date IS NOT NULL AND EWRporta2_articles.article_date < 1417987751 
OR 
EWRporta2_articles.article_date IS NOT NULL AND EWRporta2_articles.article_date IS NOT NULL AND EWRporta2_articles.article_date < 1417987751 

可以简化为：

EWRporta2_articles.article_date IS NOT NULL AND EWRporta2_articles.article_date < 1417987751 

，它可以进一步简化为：

EWRporta2_articles.article_date < 1417987751 

---

最后，我们将得到：

WHERE (
    xf_thread.node_id IN ('66','78') 
    AND 
    EWRporta2_articles.article_date IS NULL 
    AND  
    xf_thread.post_date < 1417987751 
    OR 
    EWRporta2_articles.article_date < 1417987751 
) 
    AND xf_thread.discussion_state = 'visible' 

---

现在划分查询分成两个单独的子查询，那么工会的这样的结果：

SELECT * 
FROM (
    SELECT ...... 
    JOIN ... JOIN ... JOIN ... 
    WHERE 
     xf_thread.node_id IN ('66','78') 
     AND EWRporta2_articles.article_date IS NULL 
     AND xf_thread.post_date < 1417987751 
     AND xf_thread.discussion_state = 'visible' 
    -- ORDER BY article_date DESC 
    LIMIT 0, 5 
) q1 
UNION 
SELECT * 
FROM (
    SELECT ...... 
    JOIN ... JOIN ... JOIN ... 
    WHERE 
     EWRporta2_articles.article_date < 1417987751 
     AND xf_thread.discussion_state = 'visible' 
    ORDER BY article_date DESC 
    LIMIT 0, 5 
) q2 
ORDER BY article_date DESC 
LIMIT 0, 5 

注意，在第一子查询中的条款ORDER BY article_date DESC被跳过 - 由于条件说：AND EWRporta2_articles.article_date IS NULL，那么日期总是空的，我们可以跳过冗余的排序操作，因为这是浪费时间。