How does the planner execute unions?

Hopefully a quick question.

In 7.3.4, how does the planner execute a query with union alls in it?

Does it execute the unions serially, or does it launch a "thread" for
each union (or maybe something else entirely).

Thanks,

Chris

Here is an explain from the view I'm thinking about, how does postgres
run this query?
hmd=# explain select count(1) from clmhdr where hdr_user_id = 'user_id';
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=42.48..42.48 rows=1 width=924)
-> Subquery Scan clmhdr (cost=0.00..42.41 rows=30 width=924)
-> Append (cost=0.00..42.41 rows=30 width=924)
-> Subquery Scan "*SELECT* 1" (cost=0.00..7.07 rows=5
width=924)
-> Index Scan using
clmhdr_live_hdr_user_id_hdr_clm_status_idx on clmhdr_live
(cost=0.00..7.07 rows=5 width=924)
Index Cond: (hdr_user_id =
'user_id'::character varying)
-> Subquery Scan "*SELECT* 2" (cost=0.00..7.07 rows=5
width=924)
-> Index Scan using
clmhdr_2003_hdr_user_id_hdr_clm_status_idx on clmhdr_2003
(cost=0.00..7.07 rows=5 width=924)
Index Cond: (hdr_user_id =
'user_id'::character varying)
-> Subquery Scan "*SELECT* 3" (cost=0.00..7.07 rows=5
width=924)
-> Index Scan using
clmhdr_2004_hdr_user_id_hdr_clm_status_idx on clmhdr_2004
(cost=0.00..7.07 rows=5 width=924)
Index Cond: (hdr_user_id =
'user_id'::character varying)
-> Subquery Scan "*SELECT* 4" (cost=0.00..7.07 rows=5
width=924)
-> Index Scan using
clmhdr_2005_hdr_user_id_hdr_clm_status_idx on clmhdr_2005
(cost=0.00..7.07 rows=5 width=924)
Index Cond: (hdr_user_id =
'user_id'::character varying)
-> Subquery Scan "*SELECT* 5" (cost=0.00..7.07 rows=5
width=924)
-> Index Scan using
clmhdr_2006_hdr_user_id_hdr_clm_status_idx on clmhdr_2006
(cost=0.00..7.07 rows=5 width=924)
Index Cond: (hdr_user_id =
'user_id'::character varying)
-> Subquery Scan "*SELECT* 6" (cost=0.00..7.07 rows=5
width=924)
-> Index Scan using
clmhdr_2007_hdr_user_id_hdr_clm_status_idx on clmhdr_2007
(cost=0.00..7.07 rows=5 width=924)
Index Cond: (hdr_user_id =
'user_id'::character varying)
(21 rows)

hmd=#