Re: New GUC to sample log queries

From: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
To: Adrien Nayrat <adrien(dot)nayrat(at)anayrat(dot)info>
Cc: PostgreSQL mailing lists <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: New GUC to sample log queries
Date: 2018-05-31 01:34:19
Message-ID: CAKJS1f9T6QfJSWvVzYJOBV+AozTh3Kgtk2-YmAga6qLNcGfz2g@mail.gmail.com
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On 31 May 2018 at 06:44, Adrien Nayrat <adrien(dot)nayrat(at)anayrat(dot)info> wrote:
> Here is a naive SELECT only bench with a dataset which fit in ram (scale factor
> = 100) and PGDATA and log on a ramdisk:
> shared_buffers = 4GB
> seq_page_cost = random_page_cost = 1.0
> logging_collector = on (no rotation)

It would be better to just: SELECT 1; to try to get the true overhead
of the additional logging code.

> I don't know the cost of random() call?

It's probably best to test in Postgres to see if there's an overhead
to the new code. It may be worth special casing the 0 and 1 case so
random() is not called.

+ (random() < log_sample_rate * MAX_RANDOM_VALUE);

this should be <=, or you'll randomly miss logging a query when
log_sample_rate is 1.0 every 4 billion or so queries.

Of course, it would be better if we had a proper profiler, but I can
see your need for this. Enabling logging of all queries in production
is currently reserved for people with low traffic servers and the
insane.

--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services

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