Asynchronous querying and tabular data

[Kona Andrews]

> I am afraid you have lost me here. I see no reason to infer that queryData is 
> some sort of estimate on the work required to do the real thing. In SIA it is 
> a query and returns the query result. It happens that the query result itself 
> describes something else and one column (hopefully) contains a URL to the 
> something else. It is not an estimate. 

I'm no expert on this subject, but I know that producing a meaningful 
estimate for an arbitrary SQL query is a non-trivial task, support for 
which is somewhat RDBMS-dependent. 

For those trying to provide generic data services that can operate 
as a front end for a wide range of back-end RDBMS systems, it is 
potentially impossible to produce an interoperable and meaningful 
query-estimate implementation.

Additionally, TAP might well be a useful access mechanism for "custom"
tabular data services with something other than an RDBMS in the back
end;  in these cases there might be no estimation functionality 
available at all.

In other words, support for query estimation isn't necessarily
universal in data services.  How would the queryData/stageData 
metaphor work in the situation where the service (for whatever 
reason) does not support query estimation?  

Cheers,
Kona
-- 
Kona Andrews        kea at roe.ac.uk
AstroGrid Project   http://www.astrogrid.org
IfA, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ