array_agg() does not stop aggregating according to HAVING clause

Hello list,

I have a query that goes through *billions* of rows and for the columns
that have an infrequent "datatag" (HAVING count(test_datatag_n)<10) it
selects all the IDs of the entries (array_agg(run_n)). Here is the full
query:

INSERT INTO infrequent_datatags_in_this_chunk
SELECT datatag, datatags.datatag_n, array_agg(run_n)
FROM runs_raw
JOIN datatags USING(datatag_n)
WHERE workitem_n >= 295
AND workitem_n < 714218
AND datatag IS NOT NULL
GROUP BY datatags.datatag_n
HAVING count(datatag_n) < 10
AND count(datatag_n) > 0 -- Not really needed because of the JOIN above
;

The runs_raw table has run_n as the primary key id, and an index on
workitem_n. The datatags table is a key value store with datatag_n as
primary key.

The problem is that this is extremely slow (5 hours), most likely because
it creates tens of gigabytes of temporary files as I see in the logs. I
suspect that it is writing to disk the array_agg(run_n) of all entries and
not only those HAVING count(datatag_n)<10. (I might be wrong though, as
this is only an assumption based on the amount of data written; I don't
know of any way to examine the temporary files written). While this query
is going through billions of rows, the ones with infrequent datatags are
maybe 10M.

How do I tell postgres to stop aggregating when count>=10?

Thank you in advance,
Dimitris