[zaarn]

PG isn't usually faster than MySQL on any naive database (which is 90% of database in the world I suppose)

Most E-Shops will be fine running MySQL or MariaDB.

The one thing PG excels at however is that you can tune it much more to your workload and it allows tuning the workload much more finely than MySQL/MariaDB. That and the ability to extend PG arbitrarily (try adding native functions to mysql without recompiling) via the C-FFI offered. You can write and define your own index methods that let you use an index that is perfect for the workload or you can add a new data type to support a new input with validation.

You can sink a lot of work into getting the most out of a PG database, MySQL not so much. But again, for most people MySQL will provide the same (or even better) performance than PG. (I still trust PG over MySQL after MySQL nulled out all entries of a table with only NOTNULL columns after a nasty crash)