Microsoft SQL Server forum - BINARY

Author

BINARY_CHECKSUM algorithm

Orly Junior

2005-06-25, 7:23 am

Hello,

Do you know if the algorithm for the BINARY_CHECKSUM function in documented
somewhere?
I would like to use it to avoid returning some string fields from the
server.
By returning only the checksum I could lookup the string in a hashtable and
I think this could make the code more efficient on slow connections.

Thanks in advanced and kind regards,

Orly Junior

David Portas

2005-06-25, 7:23 am

I don't know where the algorithm is documented but I don't think it will
help you. There are many more possible strings than checksums so there isn't
a one-to-one correspondence between them. Unless your set of possible
strings is constrained to a small set you couldn't guarantee to translate a
checksum back into a string and you'd still have to maintain a lookup table
of strings on the client side. So if your set of strings is small and
constrained then you may as well invent your own codes. Alternatively, take
a look at Huffman Coding or one of the zip compression algorithms.

--
David Portas
SQL Server MVP
--

Erland Sommarskog

2005-06-25, 7:23 am

Orly Junior (nomail@nomail.com) writes:
> Do you know if the algorithm for the BINARY_CHECKSUM function in
> documented somewhere? I would like to use it to avoid returning some
> string fields from the server. By returning only the checksum I could
> lookup the string in a hashtable and I think this could make the code
> more efficient on slow connections.

Risky business. The checksum algorithm is fairly simple-minded. I beleive
it uses some xor mechanism. I don't have the references around right now,
but I recall that SQL Server MVP Steve Kass demonstrated how some quite
small changes could result in the same checksum.

Better in such case, to augment the table with a timestamp column. Such
a column is automatically updated every time SQL Server updates the
row. So you could store the timestamp client side, and pass that value,
if the table has the same value, there is no need for a refresh.

--
Erland Sommarskog, SQL Server MVP, esquel@sommarskog.se

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