What happens when you blend a local file with your database?
When you blend a local file with a table in your database you must to consider that:
+ You will need to have both data at the same granular level of the relationship (at least).
+ Both data must to be in the same environment to perform the data blending.
It means that even if your viz shows a consolidated information, you may be blending detailed data.
For example, you have a sales table (database) and a region information (excel file).
+ Sales (database)
+ Region (excel file)
You want to summarize the sales per region, so you have to blend both datasources by employee.
Your desired output is:
So, you drag & drop [Region] (from your local file) to Rows shelf, and [Sales] (from your database table) to Columns shelf.
At this point, you are asking Tableau to retrieve two fields from your database: employee and sales.
So, your viz shows sales per region, but you had to retrieve to your computer (or Tableau Server) the data from your database at the employee level, because (as we said before):
+ You need that field (employee) to blend with your local file, even though you do not explicitly use that field in your viz;
+ Both data must to be in the same environment to perform the blend.
Now, Imagine if you have a table with millions of rows and you want to blend with your local file to see the sales by region. You will have to retrieve millions of rows from your database, move them to your computer and blend them with your local file.
If that is your case, be careful.
If you are a SAS user, that will be the same behavior when you join a dataset (.sas7bdat) with a table in a database.