ST_MemUnion_Agg vs ST_Union_Agg Behaviour

Hi, i'm trying to understand the difference in these two methods. For the below query, assume I have two thousand rows, with two unique parcels, and 1000 feature geoms for each.

```
CREATE OR REPLACE TABLE non_buildable_per_parcel AS
SELECT group_uuid
, ST_Difference(
ANY_VALUE(parcel_geom),
ST_Union_Agg(ST_Intersection(
feature_geom,
parcel_geom
))
) AS non_buildable_geometry
GROUP BY parcel.group_uuid
```

I'd expect this query to make two ST_Union_Agg calls, sort of like `ST_Union([parcel 1 feature geoms], parcel 1 geom) `and `ST_Union([parcel 2 feature geoms], parcel 2 geom)`, trying to union all the relevant features at the same time.

If i instead use ST_MemUnionAgg, will it do 1000 ST_Union calls for each parcel, iteratively building up on the result of each ST_Union call?

In practice the original approach often OOMs as I understand ST_UnionAgg does not spill to disk, and the second approach doesn't OOM but is very slow. So I wanted to understand the expected behaviours fully. Thank you