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Tableau will intelligently query the database and return only the data that is required. This depends on what dimensions, measures and filters you use. If the query is only for 100MB, then it will usually only be for 100MB.
If you use Custom SQL then that can change things, but if you're new then don't use Custom SQL until you need to.
That is only for live data connections, if you choose to use an extract, then it will query all of the fields shown. To decrease the amount of time an extract takes to complete, before you create the extract, hide all of the fields that you don't need at the time and then run your extract. This will remove all of the extra columns, then if you decide you need a column that you have previously hidden, then choose to not use the extract, show hidden fields, un hide the one you want, then re-extract.