Goals of the project

If you wonder what is this project for, here is a loose set of goals and achievements.

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Learning more about PostgreSQL internals
The project has been an occasion to learn something more about PostgreSQL internals and more advanced topics: memory allocation in the server, definition of complete arithmetic data types and their operators. This goal has been completely fulfilled.
Providing and edge on other databases
As far as I know no other database provides rational numbers nor the wide range of mathematical functions exposed by the GMP. Having them directly available into the database makes PostgreSQL a good choice in environments where these functions are of use, such as in mathematical and cryptography research.

Here instead is a list of non-goals:

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An extension to be used by everybody
For almost every user the performance offered by the !numeric data type are perfectly acceptable and the extra performance provided by !mpz may not justify the use of an external library.
Replacing PostgreSQL builtin numerical types
GMP data type are not constrained by the semantic specified by SQL, so they will hardly replace any standard data type. Furthermore the GMP (as well as the pgmp extension) is distributed with LGPL license, which is perfectly compatible with the PostgreSQL terms of distribution but likely to not be well accepted by the PostgreSQL developers who usually prefer BSD-style licenses.