Q:

Does PostgreSQL support rounding at the database level?

For example is it possible to ensure that a particular number field is always rounded to exactly three decimal places?

A:

Yes. Define the column as DECIMAL(16, 3) - see the postgres documentation for this type
The first number (16 in this case) is the maximum total number of digits in the number, the second number (3 in this case) is the number of decimal places.
Every database (I know of) supports this datatype.

