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.