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Query Against a dblink View Takes Too Long to Return

From: Dawn Hollingsworth <dmh(at)airdefense(dot)net>
To: pgsql-sql(at)postgresql(dot)org
Subject: Query Against a dblink View Takes Too Long to Return
Date: 2003-02-28 12:54:48
Message-ID: 1046436895.4488.214.camel@kaos (view raw, whole thread or download thread mbox)
Lists: pgsql-sql

PostgreSQL Version: 7.2.1
OS : Red Hat 7.3 with Kernel 2.4.18-5 and SGI_XFS

Currently we have a view defined:
CREATE VIEW db1_info_view AS
						              SELECT CAST(dblink_tok(t1.dblink_db1_info,0) AS
MACADDR)   AS id1,
textToBoolean(dblink_tok(t1.dblink_db1_info,2))              AS valid,
AS TIMESTAMPTZ) AS modify_time
						              FROM (SELECT dblink('hostaddr= port=5432
dbname=db1 user=postgres',
						                         'SELECT id1, id2, valid, modify_time FROM
db1_info') AS dblink_db1_info) AS t1;

Periodically we update information in the second database based upon the
modify time from this view. This is the query that is run from a stored
procedure. I've separated it out of the stored procedure because running
it by hand in psql has the same problem.
SELECT v.id1, v.id2, v.valid, v.modify_time 
FROM db1_info_view v LEFT OUTER JOIN db2_info d ON (v.id1 = d.id1 AND
v.id2 = d.id2) WHERE v.modify_time >= d.modify_time;

Until recently this has worked perfectly.  We started increasing the
number of records in these tables and now we're running into the problem
where the SELECT statement does not return. I'm not talking about a lot
of records. Both tables in the query only have about 9,000 records each.

I can do a SELECT COUNT(*) or SELECT * on the view and it comes back
instantly. I can SELECT * and ORDER BY modify_time and it comes back
instantly. As soon as I try to join the view to another table the query
takes 10 to 20 minutes to come back.


Nested Loop  (cost=0.00..466.51 rows=1 width=24) (actual
time=226.39..1018072.99 rows=9353 loops=1)
  ->  Subquery Scan t1  (cost=0.00..0.01 rows=1 width=0) (actual
time=225.99..345.86 rows=9353 loops=1)
        ->  Result  (cost=0.00..0.01 rows=1 width=0) (actual
time=225.98..303.29 rows=9353 loops=1)
  ->  Seq Scan on allowed_station_view av  (cost=0.00..162.53 rows=9353
width=20) (actual time=0.01..31.77 rows=9353 loops=9353)
Total runtime: 1018092.69 msec


If I create a temporary table by selecting everything from the view and
then do the join it comes back instantly.


Merge Join  (cost=69.83..526.68 rows=1000 width=40) (actual
time=109.03..247.78 rows=9353 loops=1)
  ->  Index Scan using allowed_station_view_pkey on allowed_station_view
av  (cost=0.00..426.88 rows=9353 width=20) (actual time=0.03..41.93
rows=9353 loops=1)
  ->  Sort  (cost=69.83..69.83 rows=1000 width=20) (actual
time=108.95..120.77 rows=9353 loops=1)
        ->  Seq Scan on allowed_station_view_test al  (cost=0.00..20.00
rows=1000 width=20) (actual time=0.07..26.67 rows=9353 loops=1)
Total runtime: 261.97 msec


Am I misusing dblink?

Dawn Hollingsworth
AirDefense, Inc.

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Subject: Re: Copy from a SELECT
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