Currently only way to get serial of a certificate by its id is to traverse acme_serial_certs_index table entirely as the id is in nvalue column. And certificate serial is not in any other place like acme_orders or acme_certs. Please add an extra field containing serial of the certificate in acme_certs nvalue column to fetch serial of a certificate easily.
Also there are lot of ways current DB schema can be improved. Is there any plans to migrate this from NoSQL to SQL?