  With back-in-time debuggers, developers can explore what happened before
observable failures by following infection chains back to their root causes.
While there are several such debuggers for object-oriented programming
languages, we do not know of any back-in-time capabilities at the
database-level. Thus, if failures are caused by SQL scripts or stored
procedures, developers have difficulties in understanding their unexpected
behavior.
  In this paper, we present an approach for bringing back-in-time debugging
down to the SAP HANA in-memory database. Our TARDISP debugger allows developers
to step queries backwards and inspecting the database at previous and arbitrary
points in time. With the help of a SQL extension, we can express queries
covering a period of execution time within a debugging session and handle large
amounts of data with low overhead on performance and memory. The entire
approach has been evaluated within a development project at SAP and shows
promising results with respect to the gathered developer feedback.
