Hi Brian,
QuickSight is sending a SQL query to the backend datasource for data retrieval, hence the processing depends on database.
Regarding table joins ,based on what I tested in Amazon Athena, below are examples of SQL when I refreshed data into SPICE.
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Custom SQL with joins in QuickSight > The query sent to Athena
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Accessing a view from QuickSight > The query sent to Athena ( View is defined in Athena )
Regards,
Koushik