To start things off, I’m going to discuss how Postgres actually stores data on disk. It’s interesting! It helps understand how Postgres queries your data on disk, how MVCC works and lots more that’s really useful when you’re trying to gain a deep understanding of how your database works for the purpose of fine-tuning performance. If you restart the server, Postgres will wipe clean the whole unlogged table because it will restore the database state from the WAL. However, if you copy out the raw database files, you can use the knowledge you have gained from this post to recover the contents of the data.

Source: How Postgres stores data on disk – this one’s a page turner - drew’s dev blog