SchemaSpy is a neat tool to produce visual diagrams for most relational databases.
Here's how to use it to generate schema relationship diagrams for PostgreSQL databases:
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Download the jar file from here (the current version is v6.1.0)
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Get the PostgreSQL JDBC driver (unless your installed version of java is really old, use the latest JDBC4 jar file)
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Run the command against an existing database. For most databases, the schema (-s option) we are interested in is the public one: