On Tue, Jan 25, 2005 at 10:40:15AM +0100, Pailloncy Jean-Gerard wrote: > Hi, > > My question is purely theoretical. > > I add use in my time in University some software that use "extended > type". > For each variable, we define the mandatory "classic type" as integer, > float, double array of. > And we define an optional "extended type" as the unit in the MKSA > system (Meter, Kilogram, Second, Ampere) or any other unit we would > have previously define (eg. Currency). > > This "extended type" was wonderful, because there was warning/error if > "extend type" does not match in any computation: you can not add apple > to orange. I think it's a wonderful idea. You could use a similar mechanism to implement: - Currencies (so you can't add dollars to pounds) - Timezone aware timestamps (so a time in Australia looks differet from a time in Europe) Probably much more. > I would appreciate to have such system in PostgreSQL. > > Do you think, it is feasible ? unrealistic ? > Any comment ? I think it is definitly feasable. There's been discussion before. I think the best way syntax-wise would be to extend the type system generically to have subtypes. For example currency(gbp) and siunit(A). This would simplify operators. You could create a simple add operator that checked the subtype and complained if they didn't match. A multiply operator for siunit might even return the appropriate derived unit. An advanced add unit for currency might lookup an exchange rate table. However, I think this might be a tricky (but very worthwhile) project. Maybe create a subtypes table with the columns (oid, supertypeid, subtypename) and use the oid here to identify the subtype in storage. To be complete it would need to change: - The parser to idenify the new type definitions - pg_dump to dump these types - input/output functions for these types - handle storage But with a bit of work it could be a nice project. Hope this helps, -- Martijn van Oosterhout <kleptog@xxxxxxxxx> http://svana.org/kleptog/ > Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a > tool for doing 5% of the work and then sitting around waiting for someone > else to do the other 95% so you can sue them.
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