There shouldn't really be. What do you want to achieve in custom SQL that you can't do?
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My understanding is that Custom SQL *always* executes when using any field from that datasource in your view. And so, if you have 5 tables being joined in your Custom SQL, those 5 tables will always be joined. However, when using Tableau to join those same 5 tables, Tableau is smart enough to exclude any unnecessary tables from the query if there are no fields needed from the table. This is something called "join culling".
Hope that helps.
Thank you for the response.
Will this issue matters only during live connection?
Because if I am doing a extract connection irrespective of join or custom sql a new extract dataset will form and if I am doing the reporting on that extract will this issue come?
It won't matter for the extracts as those data are already pulled out of the database into Tableau's proprietary TDE format.
Interesting info Tim - thanks for that
Thanks Tim, VERY good info to know!