Re: PG 15 (and to a smaller degree 14) regression due to ExprEvalStep size

On Thu, Feb 23, 2023 at 2:39 PM Andres Freund <andres(at)anarazel(dot)de> wrote:

> Hi,
>
> On 2023-02-23 13:56:56 -0500, Tom Lane wrote:
> > Corey Huinker <corey(dot)huinker(at)gmail(dot)com> writes:
> > > My not-ready-for-16 work on CAST( ... ON DEFAULT ... ) involved making
> > > FuncExpr/IoCoerceExpr/ArrayCoerceExpr have a safe_mode flag, and that
> > > necessitates adding a reserror boolean to ExprEvalStep for subsequent
> steps
> > > to test if the error happened.
> >
> > Why do you want it in ExprEvalStep ... couldn't it be in ExprState?
> > I can't see why you'd need more than one at a time during evaluation.
>
> I don't know exactly what CAST( ... ON DEFAULT ... ) is aiming for - I
> guess
> it wants to assign a different value when the cast fails? Is the default
> expression a constant, or does it need to be runtime evaluated? If a
> const,
> then the cast steps just could assign the new value. If runtime evaluation
> is
> needed I'd expect the various coerce steps to jump to the value
> implementing
> the default expression in case of a failure.
>

The default expression is itself a cast expression. So CAST (expr1 AS
some_type DEFAULT expr2 ON ERROR) would basically be a safe-mode cast of
expr1 to some_type, and only upon failure would the non-safe cast of expr2
to some_type be executed. Granted, the most common use case would be for
expr2 to be a constant or something that folds into a constant, but the
proposed spec allows for it.

My implementation involved adding a setting to CoalesceExpr that tested for
error flags rather than null flags, hence putting it in ExprEvalStep and
ExprState (perhaps mistakenly). Copying and adapting EEOP_JUMP_IF_NOT_NULL
lead me to this:

EEO_CASE(EEOP_JUMP_IF_NOT_ERROR)
{
/* Transfer control if current result is non-error */
if (!*op->reserror)
{
*op->reserror = false;
EEO_JUMP(op->d.jump.jumpdone);
}

/* reset error flag */
*op->reserror = false;

EEO_NEXT();
}