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From: Heikki Linnakangas <[email protected]>
To: Gregory Stark <[email protected]>
Cc: Juho Saarikko <[email protected]>
Cc: [email protected]
Subject: Re: BUG #3965: UNIQUE constraint fails on long column values
Date: Wed, 20 Feb 2008 11:25:53 +0000
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>
	<[email protected]>

Gregory Stark wrote:
> "Heikki Linnakangas" <[email protected]> writes:
> 
>> As others have pointed out, CREATE UNIQUE INDEX i ON ((md5(column)) is a pretty
>> good work-around.
> 
> Unless you need cryptographic security I would not suggest using MD5. MD5 is
> intentionally designed to take a substantial amount of CPU resources to
> calculate.
> 
> Postgres's internal hash method is exposed for most data types as hashtext()
> hashfloat8(), hashint4(), etc. These functions were chosen for their
> lightweight design.
> 
> Cryptographic security is important only if you're concerned with people being
> able to intentionally create collisions. In this scenario that's probably not
> a top threat. Conceivably someone could create a denial-of-service attack
> slowing down your server by causing your indexes to become unbalanced. But it
> would be fairly challenging to engineer.

Return type of hash* functions is just 32 bits. I wonder if that's wide 
enough to avoid accidental collisions? Depends on the application of 
course...

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com

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