Tableau Server comes bundled with a customized Postgres database. There's really no method (supported) to swap it out for a different database or version. Over time the expectation is that the size of the database will increase in size and this doesn't require any management it will actually do this on its own. The version of Postgres used by Tableau Server 10.5.x would be 9.5.10. Would need to doublecheck what it is for Tableau Server 2018.1.