query from partitions

Hi.

create table device(id int);

insert into device values(1);
insert into device values(2);
.....
insert into device values(250);

create table base (
id int,
data float,
datatime timestamp,
mode int,
status int);

create table base_1 (
check ( id = 1 and datatime >= DATE '2005-01-01'
and datatime < DATE '2006-01-01' )
) INHERITS (base);

create table base_2 (
check ( id = 2 and datatime >= DATE '2005-01-01'
and datatime < DATE '2006-01-01' )
) INHERITS (base);
....
create table base_250

And
select * from base
where id in (1,2) and datatime between '2005-05-15' and '2005-05-17';
10 seconds

select * from base
where id in (select id from device where id = 1 or id = 2) and
datatime between '2005-05-15' and '2005-05-17';
10 minits

Why?

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