[Marazan]

My favourite PostgresSQL optimization was running a SELECT on a multi-billion row table before running a DELETE so that the shared_memory_buffer would be filled with the rows that would need to be deleted.

Postgres makes DELETEs single threaded, this includes the selection part of the DELETE. By running a completely separate SELECT first Postgres would multithread the SELECT and populate the cache fast. Then the single thread DELETE can operate on in-memory data and not endlessly block loading data from disk.