Description of problem:
PostgreSQL enables marking the volatility and strictness of a function, which in turn might improve the running time of queries. We currently do not take advantage of this, and it might be worthwhile to mark functions this way.
Note on compatibility: while this is specific to PostgreSQL, it should be easy to write a script that drops all occurrences of IMMUTABLE, STABLE etc. if we ever move to another database implementation.
Additional info:
Volatility
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* A function should be marked as IMMUTABLE if it doesn't change the database, and if it doesn't perform any lookups (even for database configuration values) during its operation.
* A function should be marked STABLE if it doesn't change the database, but might perform lookups (IMMUTABLE is preferable if function meets the requirements).
* A function doesn't need to be marked VOLATILE, because that's the default.
STRICTNESS
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A function should be marked STRICT if it should return NULL when it is passed a NULL argument, and then the function won't even be called if it is indeed passed a NULL argument.