Shane Borden walks through an issue you might not expect:
I’m always working with customers migrating from Oracle to PostgreSQL. One of the things in Oracle that didn’t necessarily have any additional impact other than I/O against an index was if the application executed insert statements which violated a PK constraint. Typically an exception handler was added to the code and while you could argue that the application shouldn’t do that, it typically was not something that had to be dealt with too often. However in PostgreSQL it IS something you need to be aware of.
Read on to learn why, as well as what you can do about it (other than making your INSERT operation resilient to this sort of issue).