Improving a query using a lot of inner joins to wp_postmeta, a key/value table

Question

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

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

Here's the explain output.

+----+-------------+-----------+--------+-----------------------------+----------+---------+------------------------+------+----------------------------------------------+
| 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                                  |
+----+-------------+-----------+--------+-----------------------------+----------+---------+------------------------+------+----------------------------------------------+

Wordpress schema here.

Answer 1

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.

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.

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 */

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 or entity-attribute-value store. What's going on here? A few things:

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.

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

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.

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.

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.