如何优化以下查询以及什么是（使用 where；使用连接缓冲区（块嵌套循环））与 EXPLAIN

Question

我遇到了查询问题。每当我尝试选择查询时，它需要 10 到 15 秒才能执行。以及解释中是什么（使用哪里；使用连接缓冲区（块嵌套循环））

查询是

SELECT  wp_posts.ID, post_title, post_content, wp_pvc_total.postcount,
        wp_pvc_total.postnum
    FROM  wp_posts
    LEFT JOIN  wp_term_relationships
          ON ( wp_posts.ID =
               wp_term_relationships.object_id )
    LEFT JOIN  wp_term_taxonomy
          ON ( wp_term_relationships.term_taxonomy_id =
               wp_term_taxonomy.term_taxonomy_id )
    LEFT JOIN  wp_pvc_total  ON ( wp_pvc_total.postnum = wp_posts.ID )
    WHERE  wp_posts.post_author = 630
      AND  wp_posts.post_author NOT IN(675)
      AND  wp_posts.ID != 48075
    GROUP BY  wp_posts.ID
    ORDER BY  wp_pvc_total.postcount DESC
    LIMIT  0, 9;

用解释查询

+----+-------------+-----------------------+--------+---------------------------------------------------------------------------+-------------+---------+-------------------------------------------------------+-------+--------------------------------------------------------+
| id | select_type | table                 | type   | possible_keys                                                             | key         | key_len | ref                                                   | rows  | Extra                                                  |
+----+-------------+-----------------------+--------+---------------------------------------------------------------------------+-------------+---------+-------------------------------------------------------+-------+--------------------------------------------------------+
|  1 | SIMPLE      | wp_posts              | range  | PRIMARY,post_name,type_status_date,post_parent,post_author,idx_post_title | post_author | 16      | NULL                                                  |  1682 | Using index condition; Using temporary; Using filesort |
|  1 | SIMPLE      | wp_term_relationships | ref    | PRIMARY                                                                   | PRIMARY     | 8       | marriai1_topic.wp_posts.ID                            |     1 | Using index                                            |
|  1 | SIMPLE      | wp_term_taxonomy      | eq_ref | PRIMARY                                                                   | PRIMARY     | 8       | marriai1_topic.wp_term_relationships.term_taxonomy_id |     1 | Using index                                            |
|  1 | SIMPLE      | wp_pvc_total          | ALL    | NULL                                                                      | NULL        | NULL    | NULL                                                  | 19670 | Using where; Using join buffer (Block Nested Loop)     |
+----+-------------+-----------------------+--------+---------------------------------------------------------------------------+-------------+---------+-------------------------------------------------------+-------+--------------------------------------------------------+

MySQL 版本是 5.6，Innodb 引擎和表结构是

CREATE TABLE `wp_posts` (
  `ID` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `post_author` bigint(20) unsigned NOT NULL DEFAULT '0',
  `post_date` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
  `post_date_gmt` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
  `post_content` longtext NOT NULL,
  `post_title` text NOT NULL,
  `post_excerpt` text NOT NULL,
  `post_status` varchar(20) NOT NULL DEFAULT 'publish',
  `comment_status` varchar(20) NOT NULL DEFAULT 'open',
  `ping_status` varchar(20) NOT NULL DEFAULT 'open',
  `post_password` varchar(255) NOT NULL DEFAULT '',
  `post_name` varchar(200) NOT NULL DEFAULT '',
  `to_ping` text NOT NULL,
  `pinged` text NOT NULL,
  `post_modified` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
  `post_modified_gmt` datetime NOT NULL DEFAULT '0000-00-00 00:00:00',
  `post_content_filtered` longtext NOT NULL,
  `post_parent` bigint(20) unsigned NOT NULL DEFAULT '0',
  `guid` varchar(255) NOT NULL DEFAULT '',
  `menu_order` int(11) NOT NULL DEFAULT '0',
  `post_type` varchar(20) NOT NULL DEFAULT 'post',
  `post_mime_type` varchar(100) NOT NULL DEFAULT '',
  `comment_count` bigint(20) NOT NULL DEFAULT '0',
  PRIMARY KEY (`ID`),
  KEY `post_name` (`post_name`(191)),
  KEY `type_status_date` (`post_type`,`post_status`,`post_date`,`ID`),
  KEY `post_parent` (`post_parent`),
  KEY `post_author` (`post_author`),
  FULLTEXT KEY `idx_post_title` (`post_title`)
)

和

CREATE TABLE `wp_pvc_total` (
  `id` mediumint(9) NOT NULL AUTO_INCREMENT,
  `postnum` varchar(255) NOT NULL,
  `postcount` int(11) NOT NULL DEFAULT '750',
  UNIQUE KEY `id` (`id`)
) 

Answer 1

添加INDEX(postnum, postcount)

这将使联系wp_pvc_total 的效率更高。而且是“覆盖”。

BNL 效率很高，所以我不确定这个索引会不会更好。但是似乎没有有用的索引是低效的。

在另一个问题上，KEY post_name (post_name(191)) 是有问题的。见http://mysql.rjweb.org/doc.php/limits#767_limit_in_innodb_indexes