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You will likely need to use the Readonly account available starting with Tableau Server 8.2.5.
See the following link for the more info on the readonly account: http://onlinehelp.tableausoftware.com/current/server/en-us/help.htm#adminview_postgres.htm
Also the tables needed to be joined can be determined from the data dictionary:
The load performance will likely need to be determined separately: http://onlinehelp.tableausoftware.com/current/server/en-us/help.htm#perf_record_create_server.htm