On 7/8/2014 4:47 AM, Spiros Ioannou wrote:
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
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
