explaining union

I've noted a strange behaviour on where conditions,
a simple example:

create table temp1 (col1 integer, primary key (col1));
create table temp2 (col1 integer, primary key (col1));

explain
select * from (
select * from temp1
union all
select * from temp2) x
where col1 = 1;

Subquery Scan x (cost=0.00..40.00 rows=2000 width=4)
-> Append (cost=0.00..40.00 rows=2000 width=4)
-> Subquery Scan *SELECT* 1 (cost=0.00..20.00 rows=1000 width=4)
-> Seq Scan on temp1 (cost=0.00..20.00 rows=1000 width=4)
-> Subquery Scan *SELECT* 2 (cost=0.00..20.00 rows=1000 width=4)
-> Seq Scan on temp2 (cost=0.00..20.00 rows=1000 width=4)

it seems the external condition doesn't propagate on the internal subselect..

but it's only on union (or except) statements, not a subselect issue:
infact,

explain
select * from
(select * from temp1
) x
where col1 = 1;

Index Scan using temp1_pkey on temp1 (cost=0.00..4.82 rows=1 width=4)

analyzing and/or querying much larger tables makes no difference;

is there any way to improve the first plan?

TIA,
--g.

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