The attached SQL causes a crash in 9.1. It appears to be caused by:
The patch tries to copy all of the important data out of the child
context before it's reset; but it doesn't get everything. For instance,
grouping_planner() allocates subroot.glob->paramlist, and uses it in
subsequent iterations; but that isn't copied before the context is
reset. It looks like there are other things that need copying as well,
but it wasn't immediately clear to me what the best fix is.