Asynchronous querying and tabular data

[Patrick Dowler]

On Wednesday 02 May 2007 07:35, Doug Tody wrote:
> > TAP queries may contain a column with a URL to something, but the
> > standard case is that the query result is something in its own right
> > and not generally the first of two stages of work. In this light,
> > I think it is a perfectly reasonable interpretation of typical DAL
> > style to say that queryData is a synchronous method that returns a
> > query result.
>
> Right; in a simple TAP query against a data table, probably the
> operation should be synchronous, and return the table data directly
> (and this will be enough for many queries).  If this mode is used
> for a large query, probably all we can do is truncate the result,
> or return an error.  In that case there is probably no alternative
> to a two-step process of estimation followed by a staging request.

Ok, I see where you are coming from. I think the disconnect is that everyone 
else (me included) sees TAP as a single step process which can be sync or 
async; the service would decide which to support. 

I think estimation is more or less pointless - even the RDBMS with all kinds 
of internal knowledge and statistics has a hard time chosing a good query 
plan and none of the 4-5 I have used have an estimating built in. There is 
good old "select count(*)" but that is faster if the query cost is dominated 
by delivering the rows, which is not always the case. It is more often than 
not dominated by the cost of joins (including using an index and then looking 
up a bunch of rows in the table - which has cost that scales just like a 
key-join). 


-- 

Patrick Dowler
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