| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Greg Stark <stark(at)mit(dot)edu> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, "ktm(at)rice(dot)edu" <ktm(at)rice(dot)edu>, Peter Geoghegan <peter(at)2ndquadrant(dot)com>, Greg Smith <greg(at)2ndquadrant(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, Ants Aasma <ants(at)cybertec(dot)at>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Patch: add timing of buffer I/O requests |
| Date: | 2012-04-14 02:01:16 |
| Message-ID: | 1663.1334368876@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Greg Stark <stark(at)mit(dot)edu> writes:
> On Fri, Apr 13, 2012 at 8:15 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> That's probably true, but I'm not sure it's worth worrying about -
>> one-in-four-billion is a pretty small probability.
> Is this not subject to the birthday paradox? If you have a given hash
> you're worried about a collision with then you have a
> one-in-four-billion chance. But if you have a collection of hashes and
> you're worried about any collisions then it only takes about 64k
> before there's likely a collision.
... so, if pg_stat_statements.max were set as high as 64k, you would
need to worry.
Realistically, I'm more worried about collisions due to inadequacies in
the jumble calculation logic (Peter already pointed out some risk
factors in that regard).
regards, tom lane
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