TAP: indexing

[Douglas Tody]

Hi -

More on the "indexed" attribute which came up in the discussions today.
The intended usage in TAP_SCHEMA would be to indicate that on very
large tables (catalogs usually) inclusion of such a field in a query
could greatly speed up the query.  If this is not the case, e.g., the
field is not primary in an index key, then it would not be flagged as
"indexed" in this simple fashion.  Flagging a field as indexed does
not necessarily imply a native SQL DBMS index, it could be a custom
astronomy index (e.g. HTM, whatever) instead.  Users and client
applications do not care.  This is one reason why we need this,
rather than just describing the native DBMS indexes.  We could also
do the latter of course, with a separate optional schema table, to
inform more advanced queries.  But if a table field is indexed by
itself it would be flagged as "indexed".

 	- Doug