getting table meta data (primary key, in this case)

[Eric Prud'hommeaux]

How can I, during query parsing on the server, efficiently find what
field is labeled as a primary key for a given table?

Why:
SPARQL is a graph-based query language and doesn't (ideally) worry
about how the links in the graph are constructed. The obvious mapping
for the SPARQL query
  SELECT ?apt WHERE { <Orders.id=2186> <Orders.shippingAddress> ?d .
		      ?d               <Addresses.apt>          ?apt };
is
  SELECT Addresses_0.apt
    FROM Orders AS Orders_0
    JOIN Addresses as Addresses_0
         ON Orders_0.shippingAddress=Addresses_0.id
   WHERE Orders_0.id=3183;

The missing piece of info is that the Orders.shippingAddress is a
foreign key for Addresses.id . For simplicity on the first round, I'm
assuming:
  1 tuple identifiers are "primary keys" and are one field wide
  2 tuple references are "foreign keys" and are one field wide
  3 all tuple references to a table reference the same identifier

Encountering ?d a second time in the query allows me to infer that
Orders.shippingAddress is a foreign key to Addresses. I just need to
pull "id" out of the air, or better yet, out of the table meta data.

I have only seen this done when the client asks the question, which it
does with a particular query. I'm hoping not to have to do a query
while parsing, but will if I have to. I currently share no info
between queries. (how do I do that, anyways?)

For more info on this, see
  http://www.w3.org/2005/05/22-SPARQL-MySQL/
  http://www.w3.org/2003/01/21-RDF-RDB-access/
-- 
-eric

office: +81.466.49.1170 W3C, Keio Research Institute at SFC,
                        Shonan Fujisawa Campus, Keio University,
                        5322 Endo, Fujisawa, Kanagawa 252-8520
                        JAPAN
        +1.617.258.5741 NE43-344, MIT, Cambridge, MA 02144 USA
cell:   +81.90.6533.3882

(eric@stripped)
Feel free to forward this message to any list for any purpose other than
email address distribution.

Attachment: [application/pgp-signature] Digital signature signature.asc