From: Dawn Hollingsworth <dmh@airdefense.net>
To: pgsql-sql@postgresql.org
Subject: Query Against a dblink View Takes Too Long to Return
Date: 03 Mar 2003 16:58:58 +0000
Message-ID: <1046710746.1290.3.camel@kaos> (raw)
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,
CAST(dblink_tok(t1.dblink_db1_info,1)
AS MACADDR) AS id2,
textToBoolean(dblink_tok(t1.dblink_db1_info,2)) AS valid,
CAST(dblink_tok(t1.dblink_db1_info,3)
AS TIMESTAMPTZ) AS modify_time
FROM (SELECT dblink('hostaddr=127.0.0.1 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.
NOTICE: QUERY PLAN:
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
EXPLAIN
If I create a temporary table by selecting everything from the view and
then do the join it comes back instantly.
NOTICE: QUERY PLAN:
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
EXPLAIN
Am I misusing dblink?
Dawn Hollingsworth
AirDefense, Inc.
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-sql@postgresql.org
Cc: dmh@airdefense.net
Subject: Re: Query Against a dblink View Takes Too Long to Return
In-Reply-To: <1046710746.1290.3.camel@kaos>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox