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
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!
To answer my own question: A Tableau consultant told me that the queries are sent to the DB separately and the Join is performed in Tableau.
So use it with care in case of large result sets.