Wordpress: Left join php mysql very slow

Question

I use this code for select count of posts

$select = SELECT COUNT( DISTINCT wpPost.ID )
          FROM `wp_posts` wpPost
          RIGHT JOIN `wp_woocommerce_order_itemmeta` wpOrderItem
          ON wpOrderItem.`meta_value`='6246'
          WHERE wpPost.`post_status`='wc-completed'
          GROUP BY wpPost.`ID`
          ORDER BY wpPost.`post_date` DESC
$wpdb->get_results("$select");
echo $wpdb->num_rows;

But this code is very slow and long time about 40 sec!

wp_woocommerce_order_itemmeta have 388116 records

Answer 1

One reason DISTINCT should be used only when absolutely necessary is that the result set must be sorted in order to find and remove duplicates

I am not sure why your query is taking a long time, but I have one workaround, sum count by removing distinct from query.

Arbitrarily choosing to keep the minimum ID. Also, avoid using the implicit join syntax.

Your Query should be:

SELECT COUNT( wpPost.ID ), MIN( wpPost.ID)
FROM `wp_posts` wpPost
RIGHT JOIN `wp_woocommerce_order_itemmeta` wpOrderItem
ON wpOrderItem.`meta_value`='6246'
WHERE wpPost.`post_status`='wc-completed'
GROUP BY wpPost.`ID`
ORDER BY wpPost.`post_date` DESC

Answer 2

I would start with adding the following indexes to optimize this query:

ALTER TABLE
  `wp_posts`
ADD
  INDEX `wp_posts_index_1` (`post_status`, `ID`);

ALTER TABLE
  `wp_woocommerce_order_itemmeta`
ADD
  INDEX `wp_woocommerce_order_itemmeta_index_1` (`meta_value`);