It's only saved in an Original workbook which user created join calc.
If you don't have it already, I don't know the way to recover.
In server somewhere, there might be some log, but I don't know.
Shin, thanks for your answer. If I can find the original workbook, will it be as simple as editing the data source to view the original tables and joins? That would be really simple. I hope I can find it. Too bad that meta data isn't stored with the extract so you could always discover the data lineage.