PZ Optimizer Loops Forever if ApplyRule Fails for Implementing Physical Operator

[mdr223]

During the execution of Optimizer.search_optimization_space(), when the plan is first getting scored, every task to OptimizePhysicalExpression will create two new tasks:

1. An OptimizeGroup for the input_group_id(s)
2. An OptimizePhysicalExpression to re-optimize this physical expression once the input group is optimized

The OptimizeGroup is added to the end of the new_tasks so that it will execute first, before the task to re-optimize the physical expression.

If the OptimizeGroup task fails -- or more precisely, if any task in the sub-tree of tasks which optimize that group fails -- then the optimizer will loop forever, because the call to optimize the physical expression will re-create tasks (1.) and (2.) above, and the cycle repeats forever.

We can catch this behavior by checking to see if the OptimizePhysicalExpression is being invoked for a 2nd time without the input group having been costed. If we detect this -- we should throw an exception (rather than looping forever)