Re: [INTERFACES] Type mappings

tomas(at)aura(dot)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 alone is sufficient, but users should
be able to specify their own conversion functions.

For example, a table:
create table foo (bar1 text, bar2 int4);
insert into foo values ('one', 1);
insert into foo values ('two', 2);

might be translated into Prolog as:
% foo(+string, +integer)
foo('one', 1).
foo('two', 2).

However, this is not the only way (and often not the best
way) to do the conversion. What's best should be decided
by the user.

All *I* am looking for is a neat wrapper around all those
selects that fills in some parse-tree-like structures.
I am not particularly picky about types, but I would like
the thing to run in the database backend, as opposed to
another process (huge performance advantage in my case).
I would prefer it to be in Tcl, or, failing that, in C.
Basically, I need to select a subset of data from a bunch
of tables and convert it for use by other tools, e.g.
for a data mining or a production system; and convert
their output back to SQL.

It is just one example of how a generic type mapper might
be used. Your goals might be totally different.

=alex