per the spec, subsequent requests return the same content That's not the sense in which HTTP GET is idempotent. The spec says nothing about returning the same

per the spec, subsequent requests return the same content That's not the sense in which HTTP GET is idempotent. The spec says nothing about returning the same content. In fact, servers are free to return uncacheable content that changes on every GET request, and many do so (for example, a page that contains the current time is a valid HTTP GET response). The HTTP definition of idempotence only applies to side-effects on the server. You can formally reconcile this definition with the mathematical definition as follows: an HTTP request is expected to cause some operation on the server (an operation is a function with the same domain and range). This operation takes the server from one state X to a new state Y. Since HTTP GET is supposed to be idempotent, if you make a GET request to a server in state X, it is free to transition into some state Y != X, but if you reissue the request to the server in state Y, it should remain in state Y. So basically, an IMMUTABLE function in PostgreSQL is trivially idempotent in terms of operations on the server, in that it can have no effect at all on the state of the database. But as far as mathematical properties of the function go, it does not need to be idempotent.