Once upon a time I helped manage a complex post-merger system consolidation project where two mainframe based systems were being integrated. The client hadn’t done a lot of projects like that and hired outside consultants to help with the project planning, management, and execution.
We found out quickly that a few key client staff members were extremely scarce resources. One was a senior consultant who had been brought back by the client after his retirement. He was, hands-down, THE absolute expert on the target system’s very large and very complex database. I’ll call him “Alex.”