UNION ALL and sequential scans

 I was glad to find in 8.3.7 that pg was now smart enough to use an index
 for a simple UNION ALL. But unfortunately, it's not quite there yet for
 our use case.

 Consider the following dummy tables:

 create table foo (id serial primary key, val int not null);
 create table bar (id serial primary key, val int not null);
 create table baz (id1 serial primary key, id2 int not null);

 insert into foo (val) select x from generate_series(0,10000) as x;
 insert into bar (val) select x from generate_series(0,10000) as x;
 insert into baz (id2) select x from generate_series(0,10000) as x;

 This query correctly uses the primary key indexes on foo and bar:

 explain analyze select * from baz join (
 select id, val from foo
 union all
 select id, val from bar
 ) as foobar on(baz.id2=foobar.id) where baz.id1=42;

 But if I add a constant-valued column to indicate which branch of the
 union each result came from:

 explain analyze select * from baz join (
 select id, val, 'foo'::text as source from foo
 union all
 select id, val, 'bar'::text as source from bar
 ) as foobar on(baz.id2=foobar.id) where baz.id1=42;

 All of a sudden it insists on a sequential scan (and takes 800 times as
 long to run) even when enable_seqscan is set false. Is there a good
 reason for this, or is it just a missed opportunity in the optimizer?