Is non-unicity of utype/ucds a problem? The problem: is it possible to query some IVOA database directly from UCDs or Utypes? Select * from ivoa_table from your_table where “phot:sdss.mag.r” - “phot:sdss.mag.i” between 1.5 and 2.5

What's the advantage ? ● Send generic queries without having first to ask “Give me the names, utypes and units of your_table on which I would like to make a query” (or in the case of “rich” registries retrieve this from the Registry)  from UCDs: queries from brad semantic contents  from utype: queries on catalogues related to DM ● Proposal of UFI = Unique Field Identifier  but is it really necessary ?

What's the problem? ● Assume the table has several columns corresponding to the utype “phot:sdss.mag.r” r(1'') r(3'') r(5'') and several columns corresponding to the utype “phot:sdss.mag.i” i(1'') i(3'') i(5'')  What should be the reaction of the data server?

Possible answers ● (1) Error, ambiguous column designation ● (2) Ambiguous column designation, there are 3 columns corresponding to “phot:sdss.mag.r” choose among r(1'') r(3'') r(5'') ● (3) The server makes a choice (e.g. uses r(1'')- i(1'')) and returns a result with proper explanations

(1) Just an error message ● is not satisfactory ● only solution = propagate the error to the end- user for a decision of what to do

(2) Error message suggesting alternatives ● a choice can be made by the calling application among the alternatives e.g.  choose the leftmost column  change the query for a mean of the matching columns (meaningless if units differ!) ● or just report the problem tp the calling process. n

(3) Server choice ● The data server decides the best answer e.g.  r(3'') corresponds best to the resolution  the most accurate magnitude  the maximum (or minimum or mean) value  or just the leftmost column... ● The knowledge is generally at the server side ● The returned metadata (e.g. VOTable FIELD definitions) specifies the actual colum(s) selected N

incomplete model? ● likely to have several columns of a table matching a given UCD ● but for utypes, it could also indicate that the DM to which the utype refers is incomplete  phot: data model did not take into account the aperture photometry; once the data model is complete the ambiguity disappears