MariaDB has introduced dynamic columns, and MySQL users are amazed. They can now store key/value pairs in a column and fetch them back out by key.
Sound familiar? It should, because PostgreSQL has supported HStore for years, alongside XML, and recent versions have enabled JSON for the same purpose.
And of course PostgreSQL can use functional and partial, and GIN indexes to index the content of HStore, JSON and XML columns, making them fast to query against.