[doppel]

From my experience tackling a postgres server that is under-specced, a good knowledge of file systems (block devices, NFS, SANs, etc.), OS caching and kernel settings for maximum memory and such (check the postgres docs). Ofcourse, this only becomes an issue once you have a decent amount of data that does not fit into memory - before that, postgres will keep everything running relatively smooth without much tinkering.

For postgres itself, make sure you know what all the settings in the configuration means, why they make sense 90% of the time and definitely do not make sense in 10% of the time (such as a low memory server with super-fast SSD disk arrays). And in general ofcourse a good knowledge of SQL, index usage/performance. Postgres extensions (arrays, JSON, etc.) are decent, but in my experience it's something you can get into relatively fast if you are solid on the rest.