From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Andrew Dunstan <andrew(at)dunslane(dot)net> |
Cc: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, KONDO Mitsumasa <kondo(dot)mitsumasa(at)lab(dot)ntt(dot)co(dot)jp>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Rajeev rastogi <rajeev(dot)rastogi(at)huawei(dot)com>, Mitsumasa KONDO <kondo(dot)mitsumasa(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com> |
Subject: | Re: Add min and max execute statement time in pg_stat_statement |
Date: | 2014-12-21 19:12:00 |
Message-ID: | 3276.1419189120@sss.pgh.pa.us |
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Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
> On 12/21/2014 01:23 PM, Alvaro Herrera wrote:
>> The point, I think, is that without atomic instructions you have to hold
>> a lock while incrementing the counters.
> Hmm, do we do that now?
We already have a spinlock mutex around the counter adjustment code, so
I'm not sure why this discussion is being held.
> I would like someone more versed in numerical analysis than me to
> tell me how safe using sum of squares actually is in our case.
That, on the other hand, might be a real issue. I'm afraid that
accumulating across a very long series of statements could lead
to severe roundoff error in the reported values, unless we use
a method chosen for numerical stability.
regards, tom lane
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