Re: [INTERFACES] Type mappings

tomas@aura.de wrote:
> Ummm... I wasn't implying not to cache the types -- I think
> caching is a must. But we could (for example) invalidate the
> cache whenever anything happens to the pg_types table (that
> would imply setting up a notifier (did I get the PostggreSQL
> parlance right?)
 Yes, that's a nice solution.  I don't need it right now, so I don't really care. ;) 
> > > It would be interesting to write up a type mapper which could be
> > > of some more general utility (no idea whether this would be
> > > feasible at all). I'd be willing to give it a try.
> > 
> >   It is a useful thing.  E.g. my project needs conversion between
> >   SDDF, SQL, (restricted form of) Prolog, and Jess (don't ask
> >   why--it's a long story).  I would definitely appreciate a
> >   generic type mapper.  It is feasible too.  My chicken scratch
> >   has worked so far. :)
> >   
> The question is... what do we use as the target data types? I
> could just go for the Guile data types -- but then we have
> Just Another Type Mapper, Not Useful To Anyone (TM). Maybe
> with a bit more of thought (say you can register conversion
> functions with it, depending on the PostgreSQL type name (but
> it might make sense to base our decisions on other attributes
> of the table, like the input/output functions)) we could
> get something more useful.
 You might get some ideas from SDDF: http://www-pablo.cs.uiuc.edu/Project/SDDF/SDDFOverview.htm  I think, type name
aloneis sufficient, but users should be able to specify their own conversion functions.  For example, a table: create
tablefoo (bar1 text, bar2 int4); insert into foo values ('one', 1); insert into foo values ('two', 2);  might be
translatedinto Prolog as: % foo(+string, +integer) foo('one', 1). foo('two', 2).  However, this is not the only way
(andoften not the best way) to do the conversion.  What's best should be decided by the user.  All *I* am looking for
isa neat wrapper around all those selects that fills in some parse-tree-like structures. I am not particularly picky
abouttypes, but I would like the thing to run in the database backend, as opposed to another process (huge performance
advantagein my case). I would prefer it to be in Tcl, or, failing that, in C. Basically, I need to select a subset of
datafrom a bunch of tables and convert it for use by other tools, e.g. for a data mining or a production system; and
converttheir output back to SQL.  It is just one example of how a generic type mapper might be used.  Your goals might
betotally different.  =alex