Is nonunicity of utypeucds a problem The problem

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 enduser for a decision of what to do

(2) Error message suggesting alternatives ● n calling a choice can be made by the 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.

(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

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