-- Cleaning wp_commentmeta
SELECT *
  FROM wp_commentmeta
 WHERE comment_id NOT IN (SELECT comment_id FROM wp_comments);

DELETE FROM wp_commentmeta
 WHERE comment_id NOT IN (SELECT comment_id FROM wp_comments);

SELECT *
  FROM wp_commentmeta
 WHERE meta_key LIKE '%akismet%';

DELETE FROM wp_commentmeta
 WHERE meta_key LIKE '%akismet%';

-- Cleaning wp_postmeta table
SELECT *
  FROM wp_postmeta pm 
       LEFT JOIN wp_posts wp ON wp.ID = pm.post_id
 WHERE wp.ID IS NULL;

DELETE pm
  FROM wp_postmeta pm 
       LEFT JOIN wp_posts wp ON wp.ID = pm.post_id
 WHERE wp.ID IS NULL;