Optimizing Queries Using Inverse Functions
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Applications built on top of relational databases often need to
perform conversions, change of format and other transformations. One
goal is to present the data in a form that is more appropriate to what
the user expects, which might involve changing time representation,
units, currency etc. Another one is to integrate data from tables with
potentially conflicting keys.

Many of these transformations are performed by external function calls
which, on most systems, are opaque to optimization. The performance
loss can become unacceptable, especially in data integration
scenarios, such as XML/XQuery services using relational
wrappers. Unless it is provided with functions that would inverse the
transformations, along with some semantic properties, the mediator 
would have to process the entire content of the database.

I will start by presenting a few concrete examples showing the problem
and an intuitive approach for those cases, followed by a brainstorming
session that could help identify challenges and possible solutions.