Primary key of the hit_data object

What is the primary key for the hit_data object, e.g. which field or group fields do I need to use to get the primary key: a field that uniquely identifies each hit, e.g. each record of the hit_data table?

Does it expire? Like the hit id is re-used after 30 days or something similiar? Should I concatenate it to some timestamp? Which one is recommended?

I would propose some of the following options, but I'm not sure which one would be correct. In the case of hitid_low and hitid_high, do they work only if they are concatenated?

transactionID | Adobe Analytics

hitid_low

hitid_high