Float data not replicated exactly

Why are float values not replicated exactly? For example:
33.333333333333343 -> 33.333333333333336
233.33333333333331 -> 233.33333333333334
I saw a previous post about this, but I'm not satisfied with the answer
"float is an imprecise data type". Note that I don't have this problem with
DTS, let alone backup/restore. And it isn't meerly an issue of display
precision: SQL treats these as different values in <> conditions and the
values return different checksums (which is the real problem for us).
First, as Steve Kass explains to me, when you select a float and see
something like 9.000000000010, you are seeing a decimal approximation of the
exact value stored. Even if you see the same viewable output from two
floats, you can't conclude they are the same value - only viewing the float
as a binary(8) can make sure of that. So, this makes things difficult, eg
If you insert these into a float column:
insert into T values (1234.5678901)
insert into T values (1234.5678901000000000)
And then convert to binary, these are 2 different values!!!
Replicationwise, the problem seems to be that the log reader reads the float
value and puts a call into an insert SP in distribution as usual. If you
convert the publisher's floats to binary and compare to the parameter values
in msrepl_commands (also converted to binary), they are the same 99% of the
time, but the float value seems to be occasionally different. Visually it
has 1 less dp than the original (viewable using sp_browsereplcmds), which
may be the cause.
The situation might be further complicated by different hardware issues on
publisher & subscriber, resulting in different float storages.
HTH,
Paul Ibison
|||Hi Suzanne,
Even given the imprecise nature of floating-point numbers, replication
should not be changing the binary representation of floating-point numbers
as persisted at the publisher if it is configured to use binary
parameters\native mode bcp so the discrepancies that you observed are indeed
quite strange. And since I wasn't entirely sure whether my expectations were
indeed correct, I created a small scenario replicating the numbers that you
provided below both during the inital (native mode) snapshot and as (binary
parameters) incremental changes in transactional replication and the numbers
showed up exactly the same at the subscriber. I did, however, observe a
slight discrepancy of 233.33333333333331 -> 233.33333333333329 when it is
replicated as an string-literal incremental change so I am guessing that
perhaps your publication is not setup to use binary parameters (for non-SQL
Server subscriber support?). I also notice that the numbers provided below
have 17 digits whereas SQL books online states that float has a precision of
15 so the last two digits are technically up in the air. Nevertheless, I am
not sure how such discrepancies can arise without resorting to the more
exotic explanations of differing CPU architectures\OS|CRT floating handling
between the publisher and the subscriber. It would be great if you can tell
us more about your environment so we can understand better the underlying
issues involved.
-Raymond
This posting is provided "as is" with no warranties and confers no rights.
"SuzanneJ - formerly in PSS-SQL" <SuzanneJ - formerly in
PSS-SQL@.discussions.microsoft.com> wrote in message
news:EB2F9785-0ABB-4F0A-8132-ACBC9E1CBB21@.microsoft.com...
> Why are float values not replicated exactly? For example:
> 33.333333333333343 -> 33.333333333333336
> 233.33333333333331 -> 233.33333333333334
> I saw a previous post about this, but I'm not satisfied with the answer
> "float is an imprecise data type". Note that I don't have this problem
with
> DTS, let alone backup/restore. And it isn't meerly an issue of display
> precision: SQL treats these as different values in <> conditions and the
> values return different checksums (which is the real problem for us).
sql