Delete all post meta except featured image Using SQL

I rewrote the function from the link in your question to SQL.
Keep in mind that this script will also clear the page-templates assigned to the pages.

Make a backup of the wp_postmeta table first.

DELETE pm 
FROM wp_postmeta pm LEFT JOIN wp_posts p ON pm.post_id = p.id
WHERE
    (p.post_type IN ('post', 'page') AND pm.meta_key <> '_thumbnail_id')
    OR (p.post_type="attachment" AND pm.meta_key <> '_wp_attached_file' 
        AND pm.meta_key <> '_wp_attachment_metadata')