Gene Wirchenko wrote: > >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? > > Then you get told that no change has occurred when one has. I > would call this an error. That's exactly what I thought when I read that in his book. I was thinking back to the sha1 and md5 algorithms, maybe a special crc algorithm is safe from this. > >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. > > Your error. Having a Ph.D. does not stop someone from being > wrong. > >Is there a crc function in postgresql? If not what algorithm would I > >need to use to create one in pl/pgsql? > > I think you are focusing on irrelevant minutiae. Is the > performance really that bad that you have go to odd lengths to up it? It is not for performance. It is to save time writing a lot of stored procedure code. when you hav e an updateable view with 70 values that need to be checked for changes a checksum starts to sound very appealing.
