I have been hacking around for a while trying to create some example Oracle compatibility types. Canonical examples: varchar2 and number. With the new features in 8.3 and 8.4, such as user-definable typmods and type categories, it appears to be actually possible to create a type equivalent to numeric or varchar entirely in user space. Cool.
Actually doing this, however, appears to be shockingly complicated. You need to redefine all the input/output/send/receive functions and all the cast functions and casts and then tie them all together. I don't expect that this is something a user would succeed in, and not even an experienced developer would want to type all that in. I actually had to write a script to generate all that code. So while thinking about how to make this simpler I remembered the "distinct type" feature of SQL, which works quite similarly, namely the new type has the same structure as the old type, but is a separate entity. It looks like CREATE TYPE newtype AS oldtype; This feature by itself could be quite useful, and then we could simply add something like CREATE TYPE newtype AS oldtype WITH CASTS; to copy all the casts as well, so the new type can be used in contexts where the old type could be used. There is also another possible way one might want to create a compatibility type. Instead of creating a new type, create an alias for an existing type, much like we currently have built-in mappings for int -> int4, bigint -> int8, etc. The difference here is that the type you put in is not the same as the one you get dumped out. So depending on taste and requirements, a user might want to choose the distinct type or the alias route. What do you think about adding this kind of support to PostgreSQL? Obviously, some details need to be worked out, but most of this is actually straightforward catalog manipulation. -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers