Re: A performance issue with Memoize

I haven't thought thoroughly about the fix yet.  But one way I'm
thinking is that in create_subqueryscan_plan() we can first add the
subquery's subplan_params to root->curOuterParams, and then replace
outer-relation Vars in scan_clauses afterwards.  That can make us be
able to share the same PARAM_EXEC slot for the same Var that both
belongs to the subquery's uplevel vars and to the NestLoop's
outer-relation vars.  To be concrete, something like attached.