使用到wp_postmeta（键/值表）的大量内部联接来改进查询 [英] Improving a query using a lot of inner joins to wp_postmeta, a key/value table

问题描述

我正在使用一个wordpress网站执行以下查询，但我看到这个查询正在做许多内部联接，网站需要很长时间加载和下降很多，我一直在尝试创建一个查询生成相同的结果，但没有成功

am working with a wordpress website that is performing the following query, but I see this query is doing many inner joins and the website takes long to load and goes down a lot, and I have been trying to create a query that produces the same result but with no success yet

我想知道什么是更好的方法做这个

I would like to know what could be a better way to do this

SELECT *
FROM wp_posts
INNER JOIN wp_postmeta color ON wp_posts.ID = color.post_id 
INNER JOIN wp_postmeta transmission ON wp_posts.ID = transmission.post_id 
INNER JOIN wp_postmeta model ON wp_posts.ID = model.post_id 
INNER JOIN wp_postmeta brand ON wp_posts.ID = brand.post_id 

AND color.meta_key = 'color' 
AND color.meta_value = 'red' 
AND transmission.meta_key = 'transmission' 
AND transmission.meta_value = 'auto' 
AND model.meta_key = 'model' 
AND model.meta_value = 'model' 
AND brand.meta_key = 'brand' 
AND brand.meta_value = 'brand'

AND wp_posts.post_status = 'publish'
AND wp_posts.post_type = 'car'
ORDER BY wp_posts.post_title

这里解释输出。

+----+-------------+-----------+--------+-----------------------------+----------+---------+------------------------+------+----------------------------------------------+
| id | select_type | table         | type   | possible_keys               | key      | key_len | ref                          | rows | Extra                                        |
+----+-------------+-----------+--------+-----------------------------+----------+---------+------------------------+------+----------------------------------------------+
|  1 | SIMPLE      | color         | ref    | post_id,meta_key            | meta_key | 768     | const                        |  629 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | wp_posts      | eq_ref | PRIMARY,type_status_date,ID | PRIMARY  | 8       | tmcdb.color.post_id          |    1 | Using where                                  |
|  1 | SIMPLE      | brand         | ref    | post_id,meta_key            | post_id  | 8       | tmcdb.wp_posts.ID            |    4 | Using where                                  |
|  1 | SIMPLE      | transmission  | ref    | post_id,meta_key            | post_id  | 8       | tmcdb.color.post_id          |    4 | Using where                                  |
|  1 | SIMPLE      | model         | ref    | post_id,meta_key            | post_id  | 8       | tmcdb.transmission.post_id   |    4 | Using where                                  |
+----+-------------+-----------+--------+-----------------------------+----------+---------+------------------------+------+----------------------------------------------+

$ b b

此处的Wordpress架构。

推荐答案

似乎你正在尝试获得一个结果集，每个类型 car 的帖子有一行。看起来你想显示每个汽车的各种属性在帖子中，那些被保存在 postmeta 。

It seems you are trying to obtain a result set with one row per post of type car. It seems you want to display various attributes of each car in the post, and those are stashed away in postmeta.

专业提示：从不在软件中使用 SELECT * ，除非您完全知道为什么做。特别是对于包含大量 JOIN 操作的查询， SELECT * 会返回大量无意义和冗余的列。

Pro tip: Never use SELECT * in software unless you absolutely know why you're doing it. Especially with queries containing lots of JOIN operations, SELECT * returns lots of pointless and redundant columns.

有一个查询设计窍门知道WordPress postmeta 表。如果要获取特定属性，请执行以下操作：

There's a query design trick to know for the WordPress postmeta table. If you want to get a particular attribute, do this:

 SELECT p.ID, p.post_title,
        color.meta_value AS color
   FROM wp_posts AS p
   LEFT JOIN wp_postmeta AS color ON p.ID = color.post_id AND 'color' = color.meta_key
  WHERE p.post_status = 'publish'
    AND /* etc etc */

做什么是非常重要的你试图做。需要此模式，因为 postmeta 是一种称为键/值存储的特殊类型的表。这里发生了什么？有几件事：

It's super-important to understand this pattern when doing what you're trying to do. This pattern is required because postmeta is a peculiar type of table called a key/value store. What's going on here? A few things:

1. 使用此模式，每个帖子都有一行， / code>表和 postmeta 表中的特定属性。


2. 您是 LEFT JOIN c> c> $> $>

<表的 postmeta 的别名。这里是 postmeta AS颜色。

3. 您包含 meta_key （ ON 条件下的'color'= color.meta_key ）。


4. 您在 SELECT 子句中使用别名以显示 postmeta.meta_value 适当的列名。这里是 color.meta_value AS颜色。

1. Using this pattern uou get one row for each post, with some columns from the posts table and a particular attribute from the postmeta table.
2. You are LEFT JOINing the postmeta table so you still get a row if the attribute is missing.
3. You are using an alias name for the postmeta table. Here it's postmeta AS color.
4. You are including the selector for meta_key (here it's 'color' = color.meta_key) in the ON condition of the join.
5. You are using an alias in your SELECT clause to present the postmeta.meta_value item with an appropriate column name. Here it's color.meta_value AS color.

这种模式，你可以堆叠起来，用 LEFT JOIN 操作的级联，得到很多不同的属性，如此。

Once you get used to employing this pattern, you can stack it up, with a cascade of LEFT JOIN operations, to get lots of different attributes, like so.

     SELECT wp_posts.ID, wp_posts.post_title, wp_posts.whatever,
            color.meta_value        AS color,
            transmission.meta_value AS transmission,
            model.meta_value        AS model,
            brand.meta_value        AS brand
       FROM wp_posts

  LEFT JOIN wp_postmeta  AS color 
         ON wp_posts.ID = color.post_id        AND color.meta_key='color'

  LEFT JOIN wp_postmeta  AS transmission
         ON wp_posts.ID = transmission.post_id AND transmission.meta_key='transmission'

  LEFT JOIN wp_postmeta  AS model
         ON wp_posts.ID = model.post_id        AND model.meta_key='model'

  LEFT JOIN wp_postmeta  AS  brand
         ON wp_posts.ID = brand.post_id        AND brand.meta_key='brand'

      WHERE wp_posts.post_status = 'publish'
        AND wp_posts.post_type = 'car'
   ORDER BY wp_posts.post_title

我对这个查询做了一些缩进，使得更容易看到模式。您可能更喜欢使用不同的缩进样式。

I've done a bunch of indenting on this query to make it easier to see the pattern. You may prefer a different indenting style.

很难知道您为什么在问题中遇到查询的性能问题。这可能是因为你得到一个组合爆炸与所有 INNER JOIN 操作，然后过滤。但是无论如何，您显示的查询可能没有返回任何行。

It's hard to know why you were having performance problems with the query in your question. It's possibly because you were getting a combinatorial explosion with all the INNER JOIN operations that was then filtered. But at any rate the query you showed was probably returning no rows.

如果您仍然有性能问题，请尝试在 postmeta上创建复合索引（post_id，meta_key，meta_value）列中的。如果你正在创建一个WordPress插件，这可能是在插件安装时做的工作。

If you are still having performance trouble, try creating a compound index on postmeta on the (post_id, meta_key, meta_value) columns. If you're creating a WordPress plugin, that's probably a job to do at plugin installation time.

这篇关于使用到wp_postmeta（键/值表）的大量内部联接来改进查询的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！