Try the following:
- Open Tableau Desktop
- Login to your server
- Go to the data sources tab
- Hover over the extract in question - little down arrow will appear
- Click on it to download the extract as data source
- Save the data source on your drive
- Double click downloaded data source - empty workbook will appear with this data source
- Right click the data source and click 'Edit Connection'
- You should now be able to edit the custom SQL, if there was any, or change other properties of the connection that created the extract
- After that you can re-publish the extract, etc.
If you can't see the download icon/button next to the extract name, this may be due to your permission, as set by Tableau Server administrator. There are quite a few options administrators can specify for what can be done with extract, i.e. connect, edit, view, etc.
Custom SQL can only affect what data is in the extract, not the access to extract.
We don´t have a server license. My issue was brilliantly discussed here http://community.tableau.com/thread/115972 and Richard Dodds was kind of enough to detail it here https://docs.google.com/open?id=0BxzN2zxLYNqrZTZlNjdjMTUtODhmOS00MzhiLWE1MjgtNmRkNTAwNzE2Zjhm
The issue is (with desktop) if you create a workbook, followed by an extract. Then you create a child workbook of the original (or parent) then you no longer are able to refresh the original extract via the child workbook.
You have to go and find the parent workbook used to create the original extract.
So don´t go crazy creating endless variations of your dashboards only to realize you´ve long since forgotten your patient zero workbook