[GENERAL] Problems with Greatest

Hello,

I have a query using Greatest that hangs and never returns when called with two tables.

Postgres Version: 9.6

Tables

t1{ id bigint }

t2 { id bigint }

they are sharing a sequence

the query

select greatest(max(t1.id), max(t2.id)) from t1, t2;

The purpose was to call setval on the sequence after doing a bulk data load into the database.  But this query never returns.  I have tried it with various combinations,

select greatest(max(t1.id), 6) from t1; -> This returns

select greatest(max(t1.id), 6) from t1, t2; -> This never returns.

The query does work if there is only a few hundred items between the tables but I'm importing about ~300,000 rows between the two tables.  I looked at pg_stat_activity and it says the query is active

I worked around this problem using a union all query but I'm wondering if this is a bug or I am just using greatest wrong.