Today while reviewing a site’s database to look for performance bottlenecks, I found 4,000 rows of post meta associated with a single post ID. That meta has the key _used_by and is associated with a post of type 'shop_coupon'

Turns out, WooCommerce keeps a record of all uses of a coupon against the coupon in a separate row of the post meta table. 😩

This is an old, and known issue. So hopefully it is addressed before too long.