pg.ddx.io pgsql-performance@postgresql.org mailing list archive help / color / mirror / Atom feed From: lars <lhofhansl@yahoo.com>
To: Kevin Grittner <Kevin.Grittner@wicourts.gov>
Cc: Ivan Voras <ivoras@freebsd.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database
Date: Wed, 13 Jul 2011 15:41:16 -0700
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On 07/13/2011 11:42 AM, Kevin Grittner wrote:
> So transactions without an XID *are* sensitive to
> synchronous_commit. That's likely a useful clue.
>
> How much did it help the run time of the SELECT which followed the
> UPDATE?
It has surprisingly little impact on the SELECT side:
=> set synchronous_commit = on;
=> update test set created_by = '000000000000001' where tenant =
'000000000000001';
UPDATE 3712
Time: 384.702 ms
lars=> select count(*) from test where tenant = '000000000000001' and
created_date = '2011-6-30';
count
-------
3712
(1 row)
Time: 36.571 ms
=> select count(*) from test where tenant = '000000000000001' and
created_date = '2011-6-30';
count
-------
3712
(1 row)
Time: 5.702 ms
=> select count(*) from test where tenant = '000000000000001' and
created_date = '2011-6-30';
count
-------
3712
(1 row)
Time: 5.822 ms
=> set synchronous_commit = off;
SET
Time: 0.145 ms
=> update test set created_by = '000000000000001' where tenant =
'000000000000001';
UPDATE 3712
Time: 96.227 ms
=> select count(*) from test where tenant = '000000000000001' and
created_date = '2011-6-30';
count
-------
3712
(1 row)
Time: 32.422 ms
=> select count(*) from test where tenant = '000000000000001' and
created_date = '2011-6-30';
count
-------
3712
(1 row)
Time: 6.080 ms
I tried it multiple times, and while the numbers change by 5-10ms the
relationship is the same.
The same results show when I use my JDBC code to run updates/selects as
fast as possible. When synchronous_commit is
off for the SELECTing process it seems to be slightly faster.
-- Lars
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To: pgsql-performance@postgresql.org
Cc: lhofhansl@yahoo.com, Kevin.Grittner@wicourts.gov, ivoras@freebsd.org
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database
In-Reply-To: <4E1E1F0C.70005@yahoo.com>
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