[TAP] data type for column metadata

[Douglas Tody]

Hi -

Reading through this long discussion of column data types/formats my
main observation is that we appear to be confusing data models and
data semantics (understanding precisely what the table columns refer
to) with the data access mechanism.  In the case of TAP, or SQL/ADQL
etc., what we have is a relatively low level table access mechanism.
The same thing is true of something like VOTable which is merely a
generic table container.

Neither TAP/ADQL/SQL nor VOTable understands much about the data it
contains and provides access to.  It is up to the data provider and
the client application or user to understand the detailed semantics
of the data being accessed.

Hence what we define at the data access level is limited: the native
table field name; the storage data type (VOTable or DBMS/JDBC or
native SQL or whatever; these represent different levels of precision);
possibly the format in which the quantity is encoded, unless this is
otherwise constrained, e.g., by a UTYPE or some other constraint such
as allowable formats for a timestamp, or the native format defined for
the table; the unit; and optionally, UCD and UTYPE to say something
about the type of quantity or the role the quantity plays in some
externally defined data model.

Details such as the associated coordinate system or time system are
more data model issues, and are best defined separately, by the data
model specification if one is used, by an external paper documenting
the details of the table being accessed, or some similar mechanism.

In an interface such as SSA, SIA, etc. (or possibly some future
catalog data model) where we may have a well defined data model and
we do active data model mediation we may be able to deal with such
detailed semantics, but in the case of direct access to external data
tables the interface is lower level and should be fairly transparent,
with minimal transformation of the data being accessed.

 	- Doug