MySQL has some aggregation performance over postgres. Having done a recent migration of an application two things that come to mind are:

- its case insensitive by default, which can make filtering simpler, without having to deal with a duplicate column where all values are lower/upper cased. - MySQL implements loose index scan and index skip scan, which improves performance of a number of join aggregation operations (https://wiki.postgresql.org/wiki/Loose_indexscan)