The Hasura GraphQL engine automatically generates your GraphQL schema and resolvers based on your tables/views in Postgres. You don't need to write a GraphQL schema or resolvers. See How Hasura GraphQL engine works for more details.
The Hasura console gives you UI tools that speed up your data-modelling process, or working with your existing database. The console also automatically generates migrations or metadata files that you can edit directly and check into your version control.
The Hasura GraphQL engine lets you do anything you would usually do with Postgres by giving you GraphQL over native Postgres constructs.
Hasura works with most Postgres compatible flavours.