Re: BUG #19370: PG18 returns incorrect array slice results when slice bounds depend on another array expression

I wrote:
> After re-reading that patch, I suspect an aliasing problem,
> specifically from this bit in ExecInitSubPlanExpr:

> * ... No
> * danger of conflicts with other uses of resvalue/resnull as storing and
> * using the value always is in subsequent steps.

> ExecInitExprRec(arg, state,
> &state->resvalue, &state->resnull);

Yup, that's it. The problem occurs when a SubPlan is in the
subscripts of an array reference. In that case,
ExecInitSubscriptingRef has already emitted code to load the source
array into its target resv/resnull, which might well be the
ExprState's resvalue/resnull. So it's not okay for the array
subscript calculation steps to overwrite the ExprState's
resvalue/resnull, but ExecInitSubPlanExpr thinks it can.

We *could* safely use ExecInitSubPlanExpr's target resv/resnull,
but that doesn't line up with EEOP_PARAM_SET's definition:
ExecEvalParamSet is hard-wired to store from state->resvalue/resnull.
I thought all along that that was probably too simplistic.

We could either generalize EEOP_PARAM_SET to include an explicit
specification of the source value's address, or insert some kind
of LOAD operation to copy the computed value into
state->resvalue/resnull. I don't see anything that looks like
that today, though.

regards, tom lane