I do not know the answer to this but my first check would be to look at the workbook XML and see if there is a difference in the SQL it generates.
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When Tableau creates an extract from a data connection, it issues a single query to bring all data back. For a multi-table join connection, this means that all tables are joined together in a single SELECT statement. If you wish to have multiple separate extracts for each table in your database, try creating separate data connections and using data blending to bring them together in a single visualization. You can then choose to extract each data connection independently.
Can you tell me how data is stored in an extract, is the relational model or some other model is used? Just want to understand if there is going to be any repetition of data in an extract, and is the time it will take to generate going to increase? I am working on creating row level security in my database and adding mapping tables as joins to the tableau workbook.
my main concern is Many to Many relationships in the join. I am afraid that if it is one extract, it will increase the size by a big number and also take a long time to create the extract.
I believe the extract is saved in a columnar method and not the traditional row method we're used to. I'd like to know more about this as well. Anyway, it's supposed to be faster.