I have implemented 2 data cartridges and managed the development of a third one. The hard part is if you want to get the Optimizer to make useful choices. With chemistry cartridges at least many queries are highly selective and many more are very very unselective. Communicating this to Oracle isn't all that easy. And forget about using a version earlier than 10g. The 9i version always chose to use the domain index even if there was a much betteer access path.
Its an interesting technology though and for some applications its the best way to integrate complex data with Oracle.

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