Re: cyclical redundancy checksum algorithm(s)?

"Karen Hill" <karen_hill22(at)yahoo(dot)com> wrote in message
news:1159387540(dot)543806(dot)256650(at)i3g2000cwc(dot)googlegroups(dot)com(dot)(dot)(dot)
> I just finished reading one of Ralph Kimball's books. In it he
> mentions something called a cyclical redundancy checksum (crc)
> function. A crc function is a hash function that generates a checksum.
>
> I am wondering a few things. A crc function would be extremely useful
> and time saving in determining if a row needs to be updated or not (are
> the values the same, if yes don't update, if not update). In fact
> 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.
>
> Yet what happens if there is a collision of the checksum for a row?
>
> Ralph Kimball did not mention which algorithm to use, nor how to create
> a crc function that would not have collisions. He does have a PhD,
> and a leader in the OLAP datawarehouse world, so I assume there is a
> good solution.
>
> Is there a crc function in postgresql? If not what algorithm would I
> need to use to create one in pl/pgsql?
>
> regards,
> karen
>

I generally like Kimball, but this idea sounds bogus.

Of course, if you only want your data warehouse to be right some of the
time....