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Re: cyclical redundancy checksum algorithm(s)?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Karen Hill" <karen_hill22(at)yahoo(dot)com>
Cc: pgsql-general(at)postgresql(dot)org
Subject: Re: cyclical redundancy checksum algorithm(s)?
Date: 2006-09-27 21:01:48
Message-ID: 6680.1159390908@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-general
"Karen Hill" <karen_hill22(at)yahoo(dot)com> writes:
> Ralph Kimball states that this is a way to check for changes.  You just
> have an extra column for the crc checksum.  When you go to update data,
> generate a crc checksum and compare it to the one in the crc column.
> If they are same, your data has not changed.

You sure that's actually what he said?  A change in CRC proves the data
changed, but lack of a change does not prove it didn't.

People do sometimes use this logic in connection with much wider
"summary" functions, such as an MD5 hash.  I wouldn't trust it at all
with a 32-bit CRC, and not much with a 64-bit CRC.  Too much risk of
collision.

			regards, tom lane

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Subject: Re: cyclical redundancy checksum algorithm(s)?
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