Database scheme for web log analytics
So many columns in a primary key sounds strange, at least.  That means to me that the data is not really adhering to a sensible uniqueness definition - this is more often than not like this in the case of logs.  For this reason, I wouldn't even try to find a PK here, just define indexes based on the queries used against the table.  One could call this opportunistic indexing ;)

Note that in general, in a relational models PKs are very important.  Here it is more the case the data is not really relational, only stored in a RDBMS.