Amount of clients which have seen an impression of a product before purchasing - most optimal answer for Billion-record table

That would have been valuable to know from the beginning. "for a project I am working on I was asked to solve this task" doesn't indicate this is theoretical or that you're being graded on it.

Nor does it help to open a new thread:

...that doesn't include the discussion already here.

See my comment there. The SQL Server query optimizer (and some others) generates a query plan based on columns statistics that are calculated on the actual data. For an index to be useful, the statistics of the data in its column(s) have to be selective enough for the optimizer to say "using this index is substantially cheaper" than another method. The fact that you have no data makes any discussion about useful indexes moot.

We need to close one of these threads, they're duplicates. I'll let you choose which one.

Edit: closing this one, as discussion is continuing in the other thread.