| From: | Andrew Dunstan <andrew(at)dunslane(dot)net> |
|---|---|
| To: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, 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 19:04:21 |
| Message-ID: | 549719B5.6070605@dunslane.net |
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| Lists: | pgsql-hackers |
On 12/21/2014 01:23 PM, Alvaro Herrera wrote:
> Andrew Dunstan wrote:
>
>> On my blog Peter Geoghegan mentioned something about "atomic fetch-and-add"
>> being useful here, but I'm not quite sure what that's referring to. Perhaps
>> someone can give me a pointer.
> 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? That won't work for the stddev method I was
referring to, although it could for the sum of squares method. In that
case 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. And how
about min and max? They don't look like good candidates for atomic
operations.
cheers
andrew
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