Maciek Sakrejda <m.sakrejda@xxxxxxxxx> writes: > [ a bunch of questions that boil down to: ] > Isn't full fidelity possible assuming sensible rounding semantics and > enough characters of precision? The fundamental issue is that the underlying representation is binary and so its precision limit doesn't correspond to an exact number of decimal digits. The default print format tries to hide that from you by printing only as many decimal digits as the stored format is certain to be able to reproduce, ie if you enter "1.23456" you should see that printed again, not "1.23455" or "1.23457". However, the stored value is not going to be exactly equal to "1.23456". The real difficulty is that there may be more than one storable value that corresponds to "1.23456" to six decimal digits. To be certain that we can reproduce the stored value uniquely, we have to err in the other direction, and print *more* decimal digits than the underlying precision justifies, rather than a bit less. Some of those digits are going to look like garbage to the naked eye. pg_dump cares about reproducing values exactly, and not about whether things are nice-looking, so it cranks up extra_float_digits. The JDBC driver might be justified in doing likewise, to ensure that the identical binary float value is stored on both client and server --- but that isn't even a valid goal unless you assume that the server's float implementation is the same as Java's, which is a bit of a leap of faith, even if IEEE 754 is nigh universal these days. > I think extra_float_digits is an awkward, surprising implementation > detail We could have dumbed it down to a boolean "look nice versus reproduce the value exactly" switch, but it seemed like there might be applications that could use some additional flexibility. In any case, it's not Postgres' fault that there is an issue here; it's fundamental to the use of binary rather than decimal stored values. regards, tom lane -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
