In my case.
Everybody has to agree the ways the DB is going to be accesed, why?, because this way I can ....partition the big table for the right key.
It's is not just DBA & DEV, it's
User: Because we work for them and they have the bussines requirements.
Dev: Because they will give the solution to users and give us technical requirements about DB.
DBA: Because we will make developers work works, fulfilling theirs reqs.
Sysadmin: They will give us the layout to work.
The communication route is all ways, in a meeting, understanding that the work is Bussines related, not technical.