有没有更快的方法从表中选择记录并查询其子表

Question

He everyone,

I would like to get all records from a table that are changed, but also if one of its child tables has changed.

I have a working query that does this using subqueries but it takes very long (2 minutes) to complete.

Is there any other way to accomplish this that runs faster?

Thanks! Wouter

SELECT
    wp_posts.id AS order_id,
    br_order_archive.id AS archive_id,
    wp_posts.post_date_gmt AS date,
    wp_posts.post_modified_gmt AS lastmod
FROM
    wp_posts
LEFT JOIN 
    br_order_archive ON br_order_archive.order_id = wp_posts.id 
WHERE
    wp_posts.post_type = "shop_order"
AND
    wp_posts.post_status = "wc-completed"
AND (
    br_order_archive.order_id IS NULL 
    OR wp_posts.post_modified_gmt != br_order_archive.lastmod
    OR br_order_archive.lastmod_licenses != (
        select max(wp_woocommerce_software_licenses.created)
        from wp_woocommerce_software_licenses
        where wp_posts.id = wp_woocommerce_software_licenses.order_id
        )
    OR br_order_archive.lastmod_activations != (
        select max(wp_woocommerce_software_activations.activation_time)
        from wp_woocommerce_software_activations
        LEFT JOIN wp_woocommerce_software_licenses ON wp_woocommerce_software_activations.key_id = wp_woocommerce_software_licenses.key_id 
        where wp_posts.id = wp_woocommerce_software_licenses.order_id
        )
    )
ORDER BY 
    wp_posts.ID desc

Answer 1

In the absence of the necessary debugging information, let's start with this bit....

select max(a.activation_time)
  from wp_woocommerce_software_activations a
  LEFT 
  JOIN wp_woocommerce_software_licenses l
    ON a.key_id = l.key_id
  where p.id = l.order_id
    

A LEFT JOIN on a table from which you select no columns is a strange thing, as is the correlation between the LEFT JOINed table and the posts table (p). Normally, a LEFT JOINed table would not have an equals operator in a WHERE clause. For my own sanity, I would start by rewriting this as an uncorrelated subquery, regardless of the impact on performance.