There’re difference in the wp_comments table database structure. comment_ID as primary key, and four index key. They’re comment_approved (comment_approved field), comment_post_ID (comment_post_ID field), comment_approved_date_gmt (comment_approved and comment_date_gmt fields), and comment_date_gmt (comment_date_gmt field).
If you don’t set the comment_date_gmt field as index then the comments don’t appear on comment tab of admin panel.
