Re: Questions of 'for update'

The plan corresponding to your select query is as following:

In LockRows node, the executer tries to lock each tuple which are provided by the Sort node. In the meantime, it's possible that some transaction updates a tuple (which is to be locked by the current transaction) and gets committed. These changes will be visible to the current transaction if it has a transaction isolation level lesser than REPEATABLE_READ. So, the current transaction needs to check whether the updated tuple still satisfies the qual check (in your query, there is no quals, so it always satisfies). If it satisfies, it returns the updated tuple.

Since, the sort has been performed by an earlier node, the output will no longer be sorted.