What is the difference between queries and cross data base joins?
About queries and cross-database joins
For each connection, Tableau Desktop sends independent queries to the databases in the join. The results are stored in a temporary table, in the format of an extract file.
For example, suppose you create connections to two tables, dbo.listings and reviews$. These tables are stored in two different databases, SQL Server and Excel. Tableau queries the database in each connection independently. the database performs the query and applies customizations such as filters and calculations, and Tableau stores the results for each connection in a temporary table. In this example, FQ_Temp_1 is the temporary table for the connection to SQL Server and FQ_Temp_2 is the temporary table for the connection to Excel.
Sql server table
When you perform a cross-database join, the temporary tables are joined together by Tableau Desktop. These temporary tables are necessary for Tableau to perform cross-database joins.
After the tables have been joined, “topn” filter is applied to limit the number of values shown in the data grid to the first 1,000 rows. This filter is applied to help maintain responsiveness of the data grid and the overall performance of the Data Source page.