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* How does the planner execute unions?
@ 2005-08-26 20:14 Chris Hoover <revoohc@gmail.com>
0 siblings, 1 reply; 2+ messages in thread
From: Chris Hoover @ 2005-08-26 20:14 UTC (permalink / raw)
To: pgsql-performance
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=#
^ permalink raw reply [nested|flat] 2+ messages in thread
* Re: How does the planner execute unions?
@ 2005-08-26 20:45 Bruno Wolff III <bruno@wolff.to>
parent: Chris Hoover <revoohc@gmail.com>
0 siblings, 0 replies; 2+ messages in thread
From: Bruno Wolff III @ 2005-08-26 20:45 UTC (permalink / raw)
To: Chris Hoover <revoohc@gmail.com>; +Cc: pgsql-performance
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.
^ permalink raw reply [nested|flat] 2+ messages in thread
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