ADQL XMATCH

[Alex Szalay]

Distance2/Distance4
	Or 
DistanceP (for points) / DistanceC  (for coords) ?


--Alex

-----Original Message-----
From: dal-bounces at ivoa.net [mailto:dal-bounces at ivoa.net] On Behalf Of Mark Taylor
Sent: Thursday, April 14, 2016 12:46 PM
To: dal at ivoa.net
Subject: Re: ADQL XMATCH

On Tue, 12 Apr 2016, Markus Demleitner wrote:

> But DISTANCE has been chosen in the original design of ADQL, and I'm 
> an enemy of changing thing that are not technically broken for 
> essentially aesthetic reasons.  So, my vote is for keeping DISTANCE.

But *if* (not decided yet) we're changing the form of it from 2-arg to 4-arg, that would seem like a reasonable time for a non-gratuitous change of the name.

Which thought nudges me to think about the consequences of overloaded geometry functions for TAPRegExt.

TAPRegExt 1.0 sec 2.3 says this about declaring geometry functions:

    ivo://ivoa.net/std/TAPRegExt#features-adqlgeo
       Each feature declares support for one of the geometry functions
       defined by ADQL (support for these functions is in general optional
       for ADQL implementations, though TAP imposes some constraints on
       what combinations of support are permitted).

       The signature of these functions, where supported, is fixed
       by ADQL; the content of the form element is just the name of
       the function.

       Example:

       <feature>
         <form>CONTAINS</form>
       </feature>

but if we're talking about overloads, there is no uniquely fixed signature for these functions.  In other words, TAPRegExt as currently defined does not have enough syntax to declare selective support for overloaded standard geometry functions - a TAP service can't declare that it supports DISTANCE(POINT,POINT) but not DISTANCE(REAL,REAL,REAL,REAL).  (note this doesn't apply to user-defined functions, since ivo://ivoa.net/std/TAPRegExt#features-udf
does require declaration of signatures).

Does it matter?

--
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/