改进 WordPress 主题中的 MySQL Select 语句

Question

我正在使用 Industrialthemes 的 WordPress 主题引擎，发现首页的渲染使用了很多查询，这些查询在我的 MySQL 数据库中运行大约需要 0.4 秒。喜欢这个：

SELECT SQL_NO_CACHE SQL_CALC_FOUND_ROWS wp_posts.ID
FROM wp_posts
INNER JOIN wp_term_relationships ON (wp_posts.ID = wp_term_relationships.object_id)
WHERE 1 =1
  AND (wp_term_relationships.term_taxonomy_id IN (1))
  AND wp_posts.post_type = 'post'
  AND (wp_posts.post_status = 'publish'
       OR wp_posts.post_status = 'closed')
GROUP BY wp_posts.ID
ORDER BY wp_posts.post_date DESC 
LIMIT 0,  5;

有什么办法可以改进这个查询吗？据我所见，WordPress 安装为所有涉及的字段设置了默认索引。我在调优 SQL Select 语句方面的知识并不好，所以我希望一些专家能在这方面帮助我。谢谢。

（来自评论）

CREATE TABLE wp_term_relationships (
    object_id bigint(20) unsigned NOT NULL DEFAULT '0', 
    term_taxonomy_id bigint(20) unsigned NOT NULL DEFAULT '0', 
    term_order int(11) NOT NULL DEFAULT '0', 
    PRIMARY KEY (object_id,term_taxonomy_id), 
    KEY term_taxonomy_id (term_taxonomy_id), 
    KEY idx1 (object_id)
) ENGINE=MyISAM DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci

（稍后...）

CREATE TABLE wp_term_relationships (
    object_id bigint(20) unsigned NOT NULL DEFAULT '0', 
    term_taxonomy_id bigint(20) unsigned NOT NULL DEFAULT '0', 
    term_order int(11) NOT NULL DEFAULT '0', 
    PRIMARY KEY (object_id,term_taxonomy_id), 
    KEY term_taxonomy_id (term_taxonomy_id), 
    KEY idx1 (term_taxonomy_id,object_id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci

Answer 1

wp_term_relationships needs INDEX(term_taxonomy_id, object_id) -- in this order
wp_posts might benefit from INDEX(post_type, ID, post_status, post_date) -- in this order

两者都是“覆盖”索引。
前者让JOIN 有效地工作并且让优化器可以选择从wp_term_relationships 开始。它应该替换KEY term_taxonomy_id (term_taxonomy_id)。
无论先选择哪张桌子，后者都应该可以正常工作。

（更多）

SELECT  SQL_NO_CACHE SQL_CALC_FOUND_ROWS p.ID
    FROM  wp_posts AS p
     WHERE p.post_type = 'post'
      AND  p.post_status IN ( 'publish', 'closed' )
      AND EXISTS ( SELECT 1 FROM wp_term_relationships AS tr
                     WHERE p.ID = tr.object_id
                       AND tr.term_taxonomy_id IN (1) )
    ORDER BY  p.post_date DESC
    LIMIT  0, 5;

有了这个公式，

如果 EXPLAIN 以 p 开头：

p:  (post_date, post_type, post_status, ID)
p:  (post_type, post_status, ID, post_date)
tr:  (object_id, term_taxonomy_id)  -- which you have

如果 EXPLAIN 以 tr 开头：

p:  (ID)  -- which you probably have
tr:  (term_taxonomy_id, object_id)

主要问题：

• GROUP BY 正在增加努力。 （我通过将 JOIN 更改为 EXISTS 来消除它。）
• IN ( 'publish', 'closed' ) -- 禁止有效使用索引。
• SQL_CALC_FOUND_ROWS -- 表示当它到达 5 行时它不能停止。
• IN (1) 变成= 1，没问题；但是IN (1,2) 更乱。

或者，更直接地说，WP 还没有被设计成规模化。

请添加索引并获取EXPLAIN SELECT。

来自 pastebin：

SELECT  SQL_NO_CACHE p.ID
    FROM  wp_posts AS p
    WHERE  p.post_type = 'post'
      AND  p.post_status = 'publish'
      AND  EXISTS 
    (
        SELECT  1
            FROM  wp_term_relationships AS tr
            WHERE  p.ID = tr.object_id
              AND  EXISTS 
            (
                SELECT  1
                    from  wp_term_taxonomy AS tt
                    WHERE  tr.term_taxonomy_id = tt.term_taxonomy_id
                      AND  tt.taxonomy = 'post_tag'
                      AND  tt.term_id IN (548, 669) ) 
    );

这是一个不同的查询。它也需要这个：

tt:  INDEX(term_taxonomy_id, taxonomy,  -- in either order
           term_id)   -- last

还有……

SELECT  SQL_NO_CACHE wp_posts.ID
    FROM  wp_posts
    INNER JOIN  wp_term_relationships tr 
           ON (wp_posts.ID = tr.object_id)
    INNER JOIN  wp_term_taxonomy tt
           ON (tr.term_taxonomy_id = tt.term_taxonomy_id)
    WHERE  ( post_type = 'post'
              AND  post_status = 'publish'
              AND  tt.taxonomy = 'post_tag'
              AND  tt.term_id IN (548, 669)  
           )
    GROUP BY  wp_posts.ID;

需要

tt:  INDEX(taxonomy, term_id, term_taxonomy_id)  -- in this order

我会将这两个索引都添加到tt，看看EXPLAINs 和性能会发生什么变化。

重写查询看看这是否给了你“正确”的答案：

SELECT  p.ID, p.post_name, p.post_title,
        p.post_type, p.post_status,
        tt.term_id as termid, tt.taxonomy
    FROM  wp_posts AS p
    INNER JOIN  wp_term_relationships tr  ON (p.ID = tr.object_id)
    INNER JOIN  wp_term_taxonomy tt  ON (tr.term_taxonomy_id = tt.term_taxonomy_id)
    WHERE  p.post_type = 'post'
      AND  p.post_status = 'publish'
      AND  tt.taxonomy = 'post_tag'
      AND  tt.term_id IN (548, 669)
    ORDER BY  p.ID;

注意事项：

• GROUP BY 已移除
• AND/OR 可能无法按预期工作：a AND b OR c 等同于 (a AND b) OR c，但我认为您想要 a AND (b OR c)
• 您是否添加了推荐的索引？