- From: Eric Prud'hommeaux <eric@w3.org>
- Date: Thu, 6 May 2004 04:34:02 +0900
- To: Pat Hayes <phayes@ihmc.us>
- Cc: "Seaborne, Andy" <andy.seaborne@hp.com>, RDF Data Access Working Group <public-rdf-dawg@w3.org>
On Wed, May 05, 2004 at 01:30:04PM -0500, Pat Hayes wrote: > >Re-execution is much, much cheaper, essentially as there are so few > >negative > >search branches to follow. > > But why would one ever need to re-execute, if you already have the > answer to the query? (what am I missing here?) It's nice from a closure perspective. One of relational calculus's claims to fame is that you query relations (tables) and you get back relations. Result re-use scenarios like fedaration (your query server actually divided the query and sent pieces to a couple other servers) and sub-selects (SELECT street FROM addresses WHERE address.id NOT IN (SELECT address FROM people)) can lean of the specification for selecting from a (materialized) table. That is, you can treat the results of your query as you would any other table. It can also be *necessary* for closure if you have a QL with disjunction. The bindings for solutions to (?who worksIn engineering || ?who reportsTo Sue) won't allow you construct statements (and use those statements in conjunctions with other results to solve a larger query). As to aggregate graphs, they will be more compact if the subgraphs for any of the solutions re-assert the same statements [2]. They also don't need a higher protocol layer to separate the graphs. Naturally, it costs to re-execute the query, but I expect the common case scenario will have a smallish result set in comparison to the initial data set and the time to perform the latter query will be insigificant. > >[1] http://www.w3.org/2003/03/rdfqr-tests/recording-query-results.html [2] http://lists.w3.org/Archives/Public/public-rdf-dawg/2004AprJun/0260 -- -eric office: +1.617.258.5741 NE43-344, MIT, Cambridge, MA 02144 USA cell: +1.857.222.5741 (eric@w3.org) Feel free to forward this message to any list for any purpose other than email address distribution.
Received on Wednesday, 5 May 2004 15:34:21 UTC