Discussion:
Largely inconsistent query execution speed, involving psql_tmp
Spiros Ioannou
2014-07-08 09:47:25 UTC
Permalink
While executing the following query through psql :

SELECT me.* FROM measurement_events me JOIN msrcs_timestamps mt ON
me.measurement_source_id=mt.measurement_source_id WHERE measurement_time >
last_update_time

there are two behaviors observed by postgresql (8.4):
1) Either the query performs lots of reads on the database and completes in
about 4 hours (that is the normal-expected behavior)
2) Either the query starts filling-up pgsql_tmp and this causes large write
I/O on the server, and the query never actually completes on a reasonable
time (we stop it after 10h).

For some strange reason, behaviour 2 is always observed when running psql
through a bash script, while behavior 1 is only observed while running psql
interactively from command line (but not always).

explain:
# explain SELECT me.* FROM measurement_events me JOIN msrcs_timestamps mt
ON me.measurement_source_id=mt.measurement_source_id WHERE
measurement_time > last_update_time;
QUERY PLAN

----------------------------------------------------------------------------------------------
Hash Join (cost=10111.78..422893652.69 rows=2958929695 width=103)
Hash Cond: (me.measurement_source_id = mt.measurement_source_id)
Join Filter: (me.measurement_time > mt.last_update_time)
-> Seq Scan on measurement_events me (cost=0.00..234251772.85
rows=8876789085 width=103)
-> Hash (cost=5733.57..5733.57 rows=350257 width=24)
-> Seq Scan on msrcs_timestamps mt (cost=0.00..5733.57
rows=350257 width=24)
(6 rows)


We have tried so far fiddling with work_mem up to 512M - no difference.
Any suggestions?



Thanks for any help,
-Spiros Ioannou
inaccess
Andy Colson
2014-07-08 15:10:49 UTC
Permalink
Post by Spiros Ioannou
SELECT me.* FROM measurement_events me JOIN msrcs_timestamps mt ON
me.measurement_source_id=mt.measurement_source_id WHERE
measurement_time > last_update_time
1) Either the query performs lots of reads on the database and completes
in about 4 hours (that is the normal-expected behavior)
2) Either the query starts filling-up pgsql_tmp and this causes large
write I/O on the server, and the query never actually completes on a
reasonable time (we stop it after 10h).
For some strange reason, behaviour 2 is always observed when running
psql through a bash script, while behavior 1 is only observed while
running psql interactively from command line (but not always).
# explain SELECT me.* FROM measurement_events me JOIN msrcs_timestamps
mt ON me.measurement_source_id=mt.measurement_source_id WHERE
measurement_time > last_update_time;
QUERY PLAN
----------------------------------------------------------------------------------------------
Hash Join (cost=10111.78..422893652.69 rows=2958929695 width=103)
Hash Cond: (me.measurement_source_id = mt.measurement_source_id)
Join Filter: (me.measurement_time > mt.last_update_time)
-> Seq Scan on measurement_events me (cost=0.00..234251772.85
rows=8876789085 width=103)
-> Hash (cost=5733.57..5733.57 rows=350257 width=24)
-> Seq Scan on msrcs_timestamps mt (cost=0.00..5733.57
rows=350257 width=24)
(6 rows)
We have tried so far fiddling with work_mem up to 512M - no difference.
Any suggestions?
Thanks for any help,
-Spiros Ioannou
inaccess
Is there any reason you don't have an index?

One, or both, of these will help:

create index measurement_events_pk on
measurement_events(measurement_source_id);

create index msrcs_timestamps_pk on msrcs_timestamps(measurement_source_id);



measurement_events has 8 billion rows, so expect it to take a while, but
its a one time cost and should _dramatically_ increase your query
performance.

-Andy
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Spiros Ioannou
2014-07-09 11:27:01 UTC
Permalink
There are indices:
table:measurement_events "measurement_events_pkey" PRIMARY KEY, btree
(measurement_source_id, measurement_time)
table:measurement_sources "measurement_sources_pkey" PRIMARY KEY, btree
(measurement_source_id)

-Spiros
Post by Andy Colson
Post by Spiros Ioannou
SELECT me.* FROM measurement_events me JOIN msrcs_timestamps mt ON
me.measurement_source_id=mt.measurement_source_id WHERE
measurement_time > last_update_time
1) Either the query performs lots of reads on the database and completes
in about 4 hours (that is the normal-expected behavior)
2) Either the query starts filling-up pgsql_tmp and this causes large
write I/O on the server, and the query never actually completes on a
reasonable time (we stop it after 10h).
For some strange reason, behaviour 2 is always observed when running
psql through a bash script, while behavior 1 is only observed while
running psql interactively from command line (but not always).
# explain SELECT me.* FROM measurement_events me JOIN msrcs_timestamps
mt ON me.measurement_source_id=mt.measurement_source_id WHERE
measurement_time > last_update_time;
QUERY PLAN
------------------------------------------------------------
----------------------------------
Hash Join (cost=10111.78..422893652.69 rows=2958929695 width=103)
Hash Cond: (me.measurement_source_id = mt.measurement_source_id)
Join Filter: (me.measurement_time > mt.last_update_time)
-> Seq Scan on measurement_events me (cost=0.00..234251772.85
rows=8876789085 width=103)
-> Hash (cost=5733.57..5733.57 rows=350257 width=24)
-> Seq Scan on msrcs_timestamps mt (cost=0.00..5733.57
rows=350257 width=24)
(6 rows)
We have tried so far fiddling with work_mem up to 512M - no difference.
Any suggestions?
Thanks for any help,
-Spiros Ioannou
inaccess
Is there any reason you don't have an index?
create index measurement_events_pk on measurement_events(
measurement_source_id);
create index msrcs_timestamps_pk on msrcs_timestamps(measurement_
source_id);
measurement_events has 8 billion rows, so expect it to take a while, but
its a one time cost and should _dramatically_ increase your query
performance.
-Andy
--
http://www.postgresql.org/mailpref/pgsql-general
Jeff Janes
2014-07-08 16:21:27 UTC
Permalink
Post by Spiros Ioannou
SELECT me.* FROM measurement_events me JOIN msrcs_timestamps mt ON
me.measurement_source_id=mt.measurement_source_id WHERE measurement_time >
last_update_time
1) Either the query performs lots of reads on the database and completes
in about 4 hours (that is the normal-expected behavior)
2) Either the query starts filling-up pgsql_tmp and this causes large
write I/O on the server, and the query never actually completes on a
reasonable time (we stop it after 10h).
For some strange reason, behaviour 2 is always observed when running psql
through a bash script, while behavior 1 is only observed while running psql
interactively from command line (but not always).
# explain SELECT me.* FROM measurement_events me JOIN msrcs_timestamps mt
ON me.measurement_source_id=mt.measurement_source_id WHERE
measurement_time > last_update_time;
QUERY PLAN
----------------------------------------------------------------------------------------------
Hash Join (cost=10111.78..422893652.69 rows=2958929695 width=103)
Hash Cond: (me.measurement_source_id = mt.measurement_source_id)
Join Filter: (me.measurement_time > mt.last_update_time)
-> Seq Scan on measurement_events me (cost=0.00..234251772.85
rows=8876789085 width=103)
-> Hash (cost=5733.57..5733.57 rows=350257 width=24)
-> Seq Scan on msrcs_timestamps mt (cost=0.00..5733.57
rows=350257 width=24)
(6 rows)
Is this plan from a situation where it would probably take 4 hours, or from
the situation where it would probably fail to complete in 10 hours?

Cheers,

Jeff
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