Re: cyclical redundancy checksum algorithm(s)?

Stepping back a bit...

Why not use an update trigger on the affected tables to record a
lastupdated timestamp value when the record is changed.

Surely this is simpler thanks computing some kind of row hash?

John

Karen Hill wrote:
> Tom Lane wrote:
>> "Karen Hill" <karen_hill22@yahoo.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.
>
>
> On page 100 in the book, "The Data Warehouse Toolkit" Second Edition,
> Ralph Kimball writes the following:
>
> "Rather than checking each field to see if something has changed, we
> instead compute a checksum for the entire row all at once.  A cyclic
> redundancy checksum (CRC) algorithm helps us quickly recognize that a
> wide messy row has changed without looking at each of its constituent
> fields."
>
> On page 360 he writes:
>
> "To quickly determine if rows have changed, we rely on a cyclic
> redundancy checksum (CRC) algorithm.   If the CRC is identical for the
> extracted record and the most recent row in the master table, then we
> ignore the extracted record.  We don't need to check every column to be
> certain that the two rows match exactly."
>
>> 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.
>>
>
>
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