On Mon, Apr 18, 2016 at 10:22:59PM -0400, Tom Lane wrote:
> Noah Misch <n...@leadboat.com> writes:
> > On Mon, Apr 18, 2016 at 09:17:46AM -0400, Tom Lane wrote:
> >> Given that it's apparently showing the results of asind as NULL ...
> > I doubt asind is returning NULL.  Here's the query, which uses a CASE to
> > report NULL if asind returns any value not on a whitelist:
> > SELECT x,
> >        CASE WHEN asind(x) IN (-90,-30,0,30,90) THEN asind(x) END AS asind,
> >        CASE WHEN acosd(x) IN (0,60,90,120,180) THEN acosd(x) END AS acosd,
> >        CASE WHEN atand(x) IN (-45,0,45) THEN atand(x) END AS atand
> > FROM (VALUES (-1), (-0.5), (0), (0.5), (1)) AS t(x);
> Oh, duh --- should have checked the query.  Yes, the most probable theory
> must be that it's returning something that's slightly off from the exact
> value.
> > I can see the benefit for atand(-0.5) and for atand(0.5), since those are
> > inexact.  Does the CASE gain us anything for asind or acosd?
> None of these are expected to be inexact.  The point of the CASE is to
> make it obvious if what's returned isn't *exactly* what we expect.

Ah, got it.

> We could alternatively set extra_float_digits to its max value and hope
> that off-by-one-in-the-last-place values would get printed as something
> visibly different from the exact result.  I'm not sure I want to trust
> that that works reliably; but maybe it would be worth printing the
> result both ways, just to provide additional info when there's a failure.

We'd have an independent problem if extra_float_digits=3 prints the same
digits for distinguishable float values, so I wouldn't mind relying on it not
to do that.  But can we expect the extra_float_digits=3 representation of
those particular values to be the same for every implementation?

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