Message-ID: <4E1E1F0C.70005@yahoo.com>
Date: Wed, 13 Jul 2011 15:41:16 -0700
From: lars <lhofhansl@yahoo.com>
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To: Kevin Grittner <Kevin.Grittner@wicourts.gov>
CC: Ivan Voras <ivoras@freebsd.org>, pgsql-performance@postgresql.org
Subject: Re: UPDATEDs slowing SELECTs in a fully cached			 database
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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