Postgres defaults to READ COMMITTED transaction isolation, where it is possible that if you perform the same SELECT twice you will get different answers due to

Postgres defaults to READ COMMITTED transaction isolation, where it is possible that if you perform the same SELECT twice you will get different answers due to a transaction committing in between. This can affect updates in some circumstances, although if you build your database so that everything is done with INSERTs then it's pretty harmless. Postgres permits you to specify the SERIALIZABLE transaction level which is complete isolation, if you care about this. MySQL does not.