Re: [HACKERS] Slow count(*) again...

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To: Robert Haas <robertmhaas@xxxxxxxxx>
Subject: Re: [HACKERS] Slow count(*) again...
From: David Wilson <david.t.wilson@xxxxxxxxx>
Date: Thu, 3 Feb 2011 21:06:18 -0800
Cc: david@xxxxxxx, Vitalii Tymchyshyn <tivv00@xxxxxxxxx>, Jon Nelson <jnelson+pgsql@xxxxxxxxxxx>, Mladen Gogala <mladen.gogala@xxxxxxxxxxx>, Andrew Dunstan <andrew@xxxxxxxxxxxx>, Bruce Momjian <bruce@xxxxxxxxxx>, Tom Lane <tgl@xxxxxxxxxxxxx>, Craig Ringer <craig@xxxxxxxxxxxxxxxxxxxxx>, "pgsql-performance@xxxxxxxxxxxxxx" <pgsql-performance@xxxxxxxxxxxxxx>
In-reply-to: <AANLkTikN14hsVVVLJ=BQdqEcpmzkLLqSbEpqx-th4Ohx@xxxxxxxxxxxxxx>
References: <201102012247.p11Ml6u02682@xxxxxxxxxx> <4D48914B.4050403@xxxxxxxxxxxx> <4D489AD1.1010105@xxxxxxxxxxx> <AANLkTiknZi97dDwm7sHqy0EL7tpNEP9sY5uPV6AN+q=u@xxxxxxxxxxxxxx> <4D499E55.4020107@xxxxxxxxxxx> <AANLkTimo2J-BagObs_V1bGaHOO9qL+kTF6s6-wZ=M3KW@xxxxxxxxxxxxxx> <AANLkTin7A_Q=Yc9Wf_5=POp2mVqmaiwqUDsFMLG68H45@xxxxxxxxxxxxxx> <4D4A7B69.2020704@xxxxxxxxx> <alpine.DEB.2.00.1102030208440.8162@xxxxxxxxxxxxxx> <AANLkTi=Thet0VZ-X6b7tek+iSLVkE27JjiKrB5D8U0Jd@xxxxxxxxxxxxxx> <alpine.DEB.2.00.1102031243190.10088@xxxxxxxxxxxxxx> <AANLkTikmChsPODbUD4V-gQ8k2yYvvjJY6b9K_29+k7nu@xxxxxxxxxxxxxx> <alpine.DEB.2.00.1102031637480.30983@xxxxxxxxxxxxxx> <AANLkTi=Kx6rRFs3Kz2m32nLKwLGv0Ee87UF8ngKhppgM@xxxxxxxxxxxxxx> <alpine.DEB.2.00.1102031732470.30983@xxxxxxxxxxxxxx> <AANLkTikN14hsVVVLJ=BQdqEcpmzkLLqSbEpqx-th4Ohx@xxxxxxxxxxxxxx>

On Thu, Feb 3, 2011 at 6:05 PM, Robert Haas <robertmhaas@xxxxxxxxx> wrote:

If you want to randomly pick 10,000 rows out of all the rows that are
going to be inserted in the table without knowing in advance how many
there will be, how do you do that?

Reservoir sampling, as the most well-known option: http://en.wikipedia.org/wiki/Reservoir_sampling

--
- David T. Wilson
david.t.wilson@xxxxxxxxx

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