Isolation levels READ UNCOMMITTED and REPEATABLE READ

I think we could allow users to set the transaction isolation level to
READ UNCOMMITTED or REPEATABLE READ and internally behave like READ
COMMITTED or SERIALIZABLE, respectively. The SQL standard seems to allow
this:

[speaking about SET TRANSACTION]

5) The isolation level of TXN is set to an implementation-defined
isolation level that will not exhibit any of the phenomena that
the explicit or implicit <level of isolation> would not exhibit,
as specified in Table 10, "SQL-transaction isolation levels and
the three phenomena".

This says that the SQL implementation can always isolate more than the
user requested, just not less.

If we did this, it would make the SQL interface more complete at little
cost.

Comments?

--
Peter Eisentraut peter_e(at)gmx(dot)net