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:
Get the PostgreSQL JDBC driver (unless your installed version of java is really old, use the latest JDBC4 jar file)
Run the command against an existing database. For most databases, the schema (-s option) we are interested in is the public one:
java -jar schemaspy-6.1.0.jar -t pgsql \ -s public -db [db name] -u [db user] -p [password] \ -host localhost -o /tmp \ -dp /path/to/JDBC/driver/postgresql-42.2.12.jar
The above example sends all the output to the /tmp folder, but that can be changed as desired.
SchemaSpy produces several different diagrams, but the main summary diagram is in: