Date: Sat, 3 Apr 2021 11:24:02 -0400
From: Bruce Momjian <bruce@momjian.us>
To: aditya desai <admad123@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>,
	Pgsql Performance <pgsql-performance@lists.postgresql.org>
Subject: Re: SELECT Query taking 200 ms on PostgreSQL compared to 4 ms on
 Oracle after migration.
Message-ID: <20210403152402.GA8049@momjian.us>
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On Sat, Apr  3, 2021 at 11:12:01AM -0400, Bruce Momjian wrote:
> On Sat, Apr  3, 2021 at 08:38:18PM +0530, aditya desai wrote:
> > Hi Justin,
> > Yes, force_parallel_mode is on. Should we set it off?
> 
> Yes.  I bet someone set it without reading our docs:
> 
> 	https://www.postgresql.org/docs/13/runtime-config-query.html#RUNTIME-CONFIG-QUERY-OTHER
> 
> -->	Allows the use of parallel queries for testing purposes even in cases
> -->	where no performance benefit is expected.
> 
> We might need to clarify this sentence to be clearer it is _only_ for
> testing.  Also, I suggest you review _all_ changes that have been made
> to the server since I am worried other unwise changes might also have
> been made.

This brings up an issue we see occasionally.  You can either leave
everything as default, get advice on which defaults to change, or study
each setting and then change defaults.  Changing defaults without study
often leads to poor configurations, as we are seeing here.

The lucky thing is that you noticed a slow query and found the
misconfiguration, but I am sure there are many servers where
misconfiguration is never detected.  I wish I knew how to improve this
situation, but user education seems to be all we can do.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  If only the physical world exists, free will is an illusion.