Hi folks,
I think I accept that Mark and Roy are right that my use case is better tackled through an identifier column in the VOTable, rather than an identifier attribute in the <TR> element, but I wanted to follow up on Rob's comment, even though in so doing I'm straying off this list's topic somewhat.
On Thu, 21 Sep 2006, Rob Seaman wrote:
> The key point is that the cross-matching
> should be done with astronomically meaningful IDs, not just indices into
> databases.
This is an important point, and what I had in mind as I wrote about an identifier in the parent database is what we do here in Edinburgh - following the SDSS - which is to give each source in our database tables an identifier which is the HTM pixel code for its position at a sufficient fine resolution that it acts as a unique ID for that source.
That satisfies Rob's criterion of being astronomically meaningful, since it encodes the source position, and should be more future-proof than an identifier which is just the row number in the database table, but it's not a standard, so it's of limited use. We've been doing some work here - which I hope will result in the deployment of a demo web service quite soon - with caching cross-match results in a database, and thereby providing a join index for future queries against the parent databases, but have had to resort to expressing the match using whatever primary key has been used in the parent database tables. In some cases, this is an astronomically meaningful ID - as with the use of high-resolution HTM IDs - but in some cases it will be just an integer row counter, which is not very robust.
It seems to me that this cross-matching of catalogues lies at the heart of what the VO is about, and many cross-matching operations will be sufficiently computationally expensive that you want to cache the results, rather than have every VO user generate them anew. But to make persisent cross-matches requires persistent IDs for the rows in the catalogues, and I don't see how to achieve this being discussed actively in the IVOA, despite the fact that it will be needed if people are to do real astronomy with the VO.
cheers
Bob Received on 2006-09-21Z18:26:35