Re: Eager aggregation, take 3

On Tue, Sep 9, 2025 at 11:20 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> Having said that, I wonder whether there's some way that we could use
> the aggtransspace property for this. For instance, for stanullfrac, we
> use values >0 to mean absolute quantities and values <0 to mean
> proportions. The current definition of aggtranspace assigns no meaning
> to values <0, and the current coding seems to assume that sizes are
> fixed regardless of how many inputs are supplied. Maybe we could
> define aggtransspace<0 to mean that the number of bytes used per input
> value is the additive inverse of the value, or something like that.

I really like this idea. Currently, aggtransspace represents an
estimate of the transition state size provided by the aggregate
definition. If it's set to zero, a default estimate based on the
state data type is used. Negative values currently have no defined
meaning. I think it makes perfect sense to reuse this field so that
a negative value indicates that the transition state data can grow
unboundedly in size.

Attached 0002 implements this idea. It requires fewer code changes
than I expected. This is mainly because that our current code uses
aggtransspace in such a way that if it's a positive value, that value
is used as it's provided by the aggregate definition; otherwise, some
heuristics are applied to estimate the size. For the aggregates that
accumulate input rows (e.g., array_agg, string_agg), I don't currently
have a better heuristic for estimating their size, so I've chosen to
keep the current logic. This won't regress anything in estimating
transition state data size.

- Richard