[purerandomness]

I recommend the book "SQL Performance Explained" by Markus Winand: https://sql-performance-explained.com/

It covers all major databases and is a good start to dive into database interna and how to interpret output from query analyzers.

Other than that, I highly recommend joining the mailing list and IRC (#postgresql on libera.chat).

Lots of valuable tricks being shared there by people with decades of experience.

[isatty]

Did freenode get renamed?

[jlokier]

Not renamed, but a number of Freenode channels and admins moved to libera.chat recently due to non-technical IRC drama / politics over "ownership".