From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Greg Stark <stark(at)mit(dot)edu>, "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 04:20:26 |
Message-ID: | CA+TgmoaVCHNZHiONk_fPzWBJOCyEFEpu_15zEp1s7qebGY6ZaA@mail.gmail.com |
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On Fri, Apr 13, 2012 at 10:01 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> 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.
Well... at 64k, you'd be very likely to have a collision. But the
whole birthday paradox thing means that there's a non-trivial
collision probability even at lower numbers of entries. Seems like
maybe we ought to be using 64 bits here...
> 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).
...especially if collisions are even more frequent than random chance
would suggest.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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