Inputs for UCDs [this part: IEEE in ADQL]

[Mark Taylor]

On Sun, 13 Oct 2019, Markus Demleitner wrote:

> (2) ADQL needs at least an isnan operator?  Me, I think that the
> semantic difference between SQL NULL and IEEE NaN is so slim that we
> probably shouldn't map it in ADQL land.  At least, it's not
> maintained in TABLEDATA and BINARY VOTable serialisations, which
> would make any possible benefit on the ADQL side questionable.
> Hence, I'd say we should discourage having NaNs in float-valued
> columns and tell people to have proper NULLs there instead.  Matters
> become murkier when ADQL grows actual support for arrays, though.
> But let's talk about that when we properly specify arrays.

The idiom "x!=x" can be used to test for NaN-ness in some languages,
that might be usable as an ADQL NaN test in absence of a specific ISNAN
operator.

> (3) Just clarify that you may get back IEEE magic values from TAP
> queries?  I guess that's more or less implied, except that, as I
> said, NaN is not reliably representable in VOTables.  If that's what
> you'd like to see, where would you make that clarification?

I take issue with "NaN is not reliably representable in VOTables".
It is, but certain processing combinations may make it hard to
distinguish between something that was a NaN and something that
was a NULL a DB table that it came from.

Mark

--
Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
m.b.taylor at bris.ac.uk +44-117-9288776  http://www.star.bris.ac.uk/~mbt/