On Tue, Jul 8, 2014 at 2:47 AM, Spiros Ioannou <sivann@xxxxxxxxxxxx> 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)
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
