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Thread: How does the planner execute unions?

How does the planner execute unions?

From
Chris Hoover
Date:
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=#

Re: How does the planner execute unions?

From
Bruno Wolff III
Date:
On Fri, Aug 26, 2005 at 16:14:18 -0400,
  Chris Hoover <revoohc@gmail.com> wrote:
> 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).

Postgres doesn't have parallel execution of parts of queries. So it is
going to do one part followed by the other part.