Reverse Engineering a Database

Introduction
Reverse Engineering is a very powerful technique for discovering the structures of databases.
It is particularly useful for operational databases for which no up-to-date documentation is available.
I have used Oracle Designer and ERWin for exactly that purpose and it was very productive.
Many Data Modelling Tools provide this facility, including ERWin and the more affordable ones, such as Datanamic's Dezign.
Sometimes the keys and relationships are not always captured to the degree that is required and a certain amount of tailoring is required to the initial solution.
But overall, Reverse Engineering is highly recommended, and is substantially better than my second alternative which is running SQL against the RDBMS System Catalogues.

We would be very pleased to hear from you if you have any experiences or suggestions that you would like to share.

This material was taken verbatim from an answer in AskMe.com provided by Jane McDonald
of MetaRecon - phone 617-548-2481, and is included because it seems to be typical of a good product in this area.

Basically, MetaRecon provides exhaustive statistical analysis of the tables or databases and does not need to know anything about the database other than table or column names. We reverse engineer the meta data from the actual source "data" and then we provide the following information. MetaRecon completely automates and integrates the process of building an EDM system.
MetaRecon employs a sophisticated reverse engineering process to allow the user to load raw data into our software and automatically receive a complete analysis of the source data. MetaRecon analyzes the source data, providing recommendations for the target database, such as primary keys, foreign keys, table splits, normalization, dimensions, measures, and views of the data.
MetaRecon uses this information to build a model of the data.
The result is the optimal target database for the EDM application.
MetaRecon automatically generates an extraction, Transformation, and Load (ETL) job for loading the target database from the source.
MetaRecon allows a user to generate the application in their familiar EDM tools, but from a single environment.
MetaRecon allows users to integrate multiple disparate systems by providing a complete understanding of the meta data and generating the ETL programs to merge the data.
Since the application is automatically generated, the speed of implementation is dramatically increased.
Because the meta data is based upon the actual source data, accuracy is 100%, avoiding the code, load, and explode phenomenon.
MetaRecon's tight integration with MetaTrans allows for integration of data from legacy systems.
The outcome is an implementation that avoids the problems commonly encountered by EDM projects.