Re: Window Function "Run Conditions"

Hi,

On 2022-03-29 15:11:52 +1300, David Rowley wrote:
> One thing which I'm not sure about with the patch is how I'm handling
> the evaluation of the runcondition in nodeWindowAgg.c. Instead of
> having ExecQual() evaluate an OpExpr such as "row_number() over (...)
> <= 10", I'm replacing the WindowFunc with the Var in the targetlist
> that corresponds to the given WindowFunc. This saves having to double
> evaluate the WindowFunc. Instead, the value of the Var can be taken
> directly from the slot. I don't know of anywhere else we do things
> quite like that. The runcondition is slightly similar to HAVING
> clauses, but HAVING clauses don't work this way.

Don't HAVING clauses actually work pretty similar? Yes, they don't have a Var,
but for expression evaluation purposes an Aggref is nearly the same as a plain
Var:

EEO_CASE(EEOP_INNER_VAR)
{
int attnum = op->d.var.attnum;

/*
* Since we already extracted all referenced columns from the
* tuple with a FETCHSOME step, we can just grab the value
* directly out of the slot's decomposed-data arrays. But let's
* have an Assert to check that that did happen.
*/
Assert(attnum >= 0 && attnum < innerslot->tts_nvalid);
*op->resvalue = innerslot->tts_values[attnum];
*op->resnull = innerslot->tts_isnull[attnum];

EEO_NEXT();
}
vs
EEO_CASE(EEOP_AGGREF)
{
/*
* Returns a Datum whose value is the precomputed aggregate value
* found in the given expression context.
*/
int aggno = op->d.aggref.aggno;

Assert(econtext->ecxt_aggvalues != NULL);

*op->resvalue = econtext->ecxt_aggvalues[aggno];
*op->resnull = econtext->ecxt_aggnulls[aggno];

EEO_NEXT();
}

specifically we don't re-evaluate expressions?

This is afaics slightly cheaper than referencing a variable in a slot.

Greetings,

Andres Freund