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.

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