<div dir="ltr"><div><div><div><div>I don't think I am the only one who is not enamored of these<br></div>brute-force unnuanced cross-matches, but I wonder whether<br></div>it would be helpful to improve them by allowing two radii.<br><br></div>Cheers,<br><br></div> - Arnold<br></div><div class="gmail_extra"><br clear="all"><div><div class="gmail_signature"><div dir="ltr">-------------------------------------------------------------------------------------------------------------<br>Arnold H. Rots Chandra X-ray Science Center<br>Smithsonian Astrophysical Observatory tel: +1 617 496 7701<br>60 Garden Street, MS 67 fax: +1 617 495 7356<br>Cambridge, MA 02138 <a href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a><br>USA <a href="http://hea-www.harvard.edu/~arots/" target="_blank">http://hea-www.harvard.edu/~arots/</a><br>--------------------------------------------------------------------------------------------------------------<br><br></div></div></div>
<br><div class="gmail_quote">On Tue, Feb 9, 2016 at 10:21 AM, Mark Taylor <span dir="ltr"><<a href="mailto:M.B.Taylor@bristol.ac.uk" target="_blank">M.B.Taylor@bristol.ac.uk</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi Marco and DAL,<br>
<br>
I support the introduction of a new XMATCH function for two main reaons:<br>
<br>
Usability:<br>
A dedicated crossmatch function should be easier<br>
for users to use and remember, and all round less horrible,<br>
than the current recommended(?) cross-match idiom:<br>
1 = CONTAINS(POINT(coordsys, lon1, lat1),<br>
CIRCLE(coordsys, lon2, lat2, radius))<br>
(where coordsys is a string that should probably be 'ICRS', or<br>
maybe should be empty, but anyway is unlikely to make much<br>
difference to the result).<br>
<br>
Implementability:<br>
It may make it easy/possible to provide standard ADQL syntax for<br>
efficient sky crossmatching in database backends that otherwise<br>
can't do it, because they have trouble implementing the ADQL<br>
geometry functions (because they lack pgSphere).<br>
<br>
In my opinion it should look like this:<br>
<br>
1 = XMATCH(lon1, lat1, lon2, lat2, radius)<br>
<br>
The alternative would presumably be<br>
<br>
1 = XMATCH(POINT(coordsys, lon1, lat1), POINT(coordsys, lon2, lat2), radius)<br>
<br>
which from the point of view of usability is not much better than<br>
the status quo. Although I believe the annoying and disingenuous<br>
coordsys argument to the geometry functions is scheduled for removal<br>
from ADQL, my understanding is that it's not intended for this<br>
(minor) revision. Even without the coordsys arg, I think the<br>
POINTless form looks less intimidating for users.<br>
<br>
I would have thought that from the point of view of implementability<br>
as well the POINTless form presents fewer constraints.<br>
However, I'm not a database implementation person, so I might be<br>
wrong about that.<br>
<br>
Mark<br>
<div><div class="h5"><br>
On Tue, 9 Feb 2016, Marco Molinaro wrote:<br>
<br>
> Dear DAL members and ADQL fans,<br>
> to go on with the ADQL-2.1 working draft<br>
> one issue is left, from Sydney interop,<br>
> to be discussed.<br>
><br>
> In the DAL splinter at the interop<br>
> it was agreed to add an XMATCH function<br>
> of binary type and definition<br>
><br>
> 1 = XMATCH(a,b,radius)<br>
><br>
> However no agreement was reached about<br>
> the 'a' and 'b' parameters, whether they<br>
> should be points (ADQL:POINT) or RA&Dec<br>
> couples (floating point values).<br>
><br>
> Both choices have advantages and disadvantages.<br>
> Points are more into the logic<br>
> of a sky cross-match but require geometric<br>
> types to be directly available to the DB.<br>
> Coordinates couples are directly available<br>
> in whatever DB and would also let the XMATCH<br>
> function work for non-orthodox coordinates<br>
> matching, but of course loosing the sky matching<br>
> logic.<br>
><br>
> As I said (also due to time constraints) no<br>
> agreement was found in Sydney.<br>
><br>
> What's your opinion on this, and why?<br>
><br>
> Cheers,<br>
> Marco<br>
><br>
<br>
</div></div>--<br>
Mark Taylor Astronomical Programmer Physics, Bristol University, UK<br>
<a href="mailto:m.b.taylor@bris.ac.uk">m.b.taylor@bris.ac.uk</a> <a href="tel:%2B44-117-9288776" value="+441179288776">+44-117-9288776</a> <a href="http://www.star.bris.ac.uk/~mbt/" rel="noreferrer" target="_blank">http://www.star.bris.ac.uk/~mbt/</a><br>
</blockquote></div><br></div>