Thomas Munro <thomas.munro@gmail.com> writes:
> Hmm, so something like a new argument "bool final" added to the
> ExecXXXShutdown() functions, which receives false in this case to tell
> it that there could be a rescan so keep the parallel context around.
I think this is going in the wrong direction. Nodes should *always*
assume that a rescan is possible until ExecEndNode is called. See the
commentary about EXEC_FLAG_REWIND in executor.h:
* REWIND indicates that the plan node should try to efficiently support
* rescans without parameter changes. (Nodes must support ExecReScan calls
* in any case, but if this flag was not given, they are at liberty to do it
* through complete recalculation. Note that a parameter change forces a
* full recalculation in any case.)
If nodeLimit is doing something that's incompatible with that, it's
nodeLimit's fault; and similarly for the parallel machinery.
If you want to do otherwise, you are going to be inventing a whole
bunch of complicated and doubtless-initially-buggy control logic
to pass down information about whether a rescan might be possible.
That doesn't sound like a recipe for a back-patchable fix. Perhaps
we could consider redesigning the rules around REWIND in a future
version, but that's not where to focus the bug fix effort.
regards, tom lane