Issues relating to simple data and metadata queries

[Patrick Dowler]

My take on this issue is that the service definitely has to know how to write 
a VOTable with it's own result metadata and clients of the service have to 
know how to parse that VOTable metadata too -- so the VOTable option is 
clearly a very low barrier to entry on both sides. This applies to the 
metadata of a query result and for that part at least I think it is clear 
that VOTable metadata should be included in the response. This makes query 
results readily usable in VOTable-aware apps like Topcat, VOPlot, etc.

Once you go past that (to metadata about what tables are available and how to 
use them) we have to spec it: which columns have indices, which columns have 
uniqueness constaints, which columns one can/should use to join two tables, 
the cardinality of the join, etc. This is not so trivial as it looks; my 
experience with maybe 3-4 different RDBMSs is that they each provide some of 
the info and other important bits are embedded in the data model 
(cardinality, for example). But you have to know these things to write decent 
queries.

For the VOResource metadata, I am not familiar with how richly one can 
describe a set of related tables. If someone with that knowledge can describe 
briefly how VOResource can or cannot handle this stuff, that would help (me 
and others, I suspect)... specifically: keys, indices (hints), join 
cardinality, that kind of stuff.

Pat

On Tuesday 01 May 2007 08:29, Doug Tody wrote:
> Considering only the synchronous part of the interface, I wonder if
> there is any possibility of agreement if we permit both VOTable and
> XML/VOResource output for metadata queries?  This would provide both
> a uniform query mechanism to make things simpler for most client
> applications, plus XML for those who have existing software which
> requires it.  So long as the information content is the same (or
> a subset), serialization to either format (or CSV for that matter)
> is pretty simple.


-- 

Patrick Dowler
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