fast refresh of MV based on a view

Hi,

I try to define a MV based on a (very simple) view. There are some problems with fast refresh (complete refresh works fine) – see below. Can anybody explain me why is fast refresh not possible? There are no sysdates, rownum1, joins, unions etc. where the fast refresh doesn't work on the documented way!
Interestingly if the MV accesses the source view over a dblink the error message is slightly different:
ORA-12014: table 'VV' does not contain a primary key constraint

Thanks,

Jaromir

Oracle9i Enterprise Edition Release 9.2.0.7.0 - 64bit

SQL> create table tt (i number primary key);

Table created.

SQL> create view vv as select * from tt;

View created.

SQL> CREATE MATERIALIZED VIEW LOG ON tt WITH primary key, rowid;

Materialized view log created.

SQL> create materialized view mm
  2 build immediate
  3 refresh fast on demand
  4 as
  5 select * from vv;
select * from vv

               *
ERROR at line 5:
ORA-12015: cannot create a fast refresh materialized view from a complex query

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