In my example, I just give each record a different ID to access it efficiently.
In our business cases, some times, we also use some prefix letter like 'SO', "PO' combining with the current year, month and then a sequence
to make a invoice ID,
for example, SO201507_1001, PO201503_1280, etc.
As these IDs would be the most important attribute to the business, so, we hope there is no gap for the IDs.
Regards,
Daojing Zhou.