1 Reply Latest reply on Dec 2, 2016 1:43 AM by Franz Graf

    Performance of Cross DB-Joins (=what is going in Tableau/DB?)

    Franz Graf

      Hi all,


      I just recently discovered that one can do cross database Joins in v10 (which is great!).

      I just wonder how this performs with large tables and how this join is done at all (DB vs Tableau). Unfortunately I didn't find accorindg posts here or on google.


      In my case I usually connect to one MySQLdatabase server holding a couple of databases. Untill now I did custom SQLs like
      "Select ...
      FROM db1.tblA ON (...)
      JOIN db2.tblB ON (...)
      ..." and the MySQL did all the rest.


      Now I can do this with 2 connections to the same server and connect the tables in the "Data Source"-Tab. But:

      - is the join still done on the MySQL side? (does tableau realize that it's the "same" connection?)

      - are the tables queries separately (one in it's own connection) and the join is done on the Tableau side?


      When I do a Tableau performance analysis (via Performance recording), the query looks like the join is done on the DB side. But as I am not a pro in interpreting the performance result it could also be that I just see the query that is performed locally on the data.


      I'd be happy if someone could shed some light on this.


      Thanks a lot!