Further thoughts on nulls in VOTables.

[Tom McGlynn]

A brief discussion we had a few weeks ago with regard to nulls in 
VOTables and TAP noted the origins of the current conventions which I 
had been blissfully ignorant of.  I remain concerned that there are 
real issues that may arise as we try to support general table ingest 
and queries using VOTables for the serialization of the data.  The 
issues I see are:

    1. Real fields in VOTables cannot distinguish nulls and NaNs
    2. The specification of nulls for strings and the mechanisms
for distinguishing null strings and 0 length strings are unclear.
    3. Integer columns may be required to confuse actual data and nulls
    4. Robust serialization of query results involving integers and
strings is incompatible with streaming results.

The attached document elaborates on these and I'd be interested in 
others' thoughts.

	Regards,
	Tom McGlynn
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