On Mon, Feb 26, 2018 at 11:29 AM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Chapman Flack <c...@anastigmatix.net> writes: > > The 0002-*.patch is a proof-of-concept patching float4_numeric and > > float8_numeric in the trivial way (just using FLT_DECIMAL_DIG and > > DBL_DECIMAL_DIG in place of FLT_DIG and DBL_DIG). It makes the new > > regression test pass. (It will only work under a compiler that has > > __FLT_DECIMAL_DIG__ and __DBL_DECIMAL_DIG__ available, and I used > > those internal versions to avoid mucking with build tooling to change > > the target C standard, which I assume wouldn't be welcome anyway. > > Nope. TBH, I'd think about just using "DBL_DIG + 3", given our existing > coding around extra_float_digits in places like pg_dump and postgres_fdw. > The knowledge that you need 2 or 3 extra digits is already well embedded. > > Conceivably you could do it like > > #ifndef DBL_DECIMAL_DIG > #ifdef __DBL_DECIMAL_DIG__ > #define DBL_DECIMAL_DIG __DBL_DECIMAL_DIG__ > #else > #define DBL_DECIMAL_DIG (DBL_DIG + 3) > #endif > #endif > > but I'm not exactly seeing how that buys us anything. > > The bigger question here is whether people actually want this behavioral > change. I think there's probably a bigger chance of complaints that > "casting 1.1::float8 to numeric now produces some weird, > incorrectly-rounded result" than that we make anyone happier. > > I have a vague idea that at some point in the past we discussed making > this conversion use extra_float_digits, which'd allow satisfying both > camps, at the nontrivial price that the conversion would have to be > considered stable not immutable. We didn't pull the trigger, if this > memory is real at all, presumably because of the mutability issue. > > Another idea would be to leave the cast alone and introduce a named > function that does the "exact" conversion. Possibly that makes nobody > happy, but at least both the cast and the function could be immutable. > It'd dodge backwards-compatibility objections, too. > > regards, tom lane > Working for a company that has enterprise customers this can't be overemphasized. Never require the user to do something so they keep getting the same results. It doesn't matter if it's "wrong". I would vote for a property. If you want the best effort to match the IEEE spec you need to execute 'set use_ieee_numbers' and you'll get the extra digits and rounding behavior. If not you'll get the existing behavior. Bear
