Date: Mon, 5 May 2014 14:51:21 -0700 (PDT)
From: David G Johnston <david.g.johnston@gmail.com>
To: pgsql-performance@postgresql.org
Message-ID: <1399326680993-5802555.post@n5.nabble.com>
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References: 
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Subject: Re: recently and selectively slow, but very simple, update query....
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Stelios Mavromichalis wrote
>> the load of the machine is also low (like 0.2).

Which means little if the update is waiting for a lock to be released by one
other process; which is more likely the situation (or some other concurrency
contention) especially as you said that this particular user generates
significant transaction/query volume (implied by the fact the user has the
most balance updates).

During slow-update executions you want to look at:
pg_stat_activity
pg_locks 

to see what other concurrent activity is taking place.

It is doubtful that dump/restore would have any effect given that the
symptoms are sporadic and we are only talking about a select statement that
returns a single row; and an update that does not hit any indexed column and
therefore benefits from "HOT" optimization.

HTH

David J.


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