Last week a i defined a new materialized view, and the on-commit was not working. After hours reading manuals, i found out that in the definition if you define FAST and ON COMMIT, fast is taken 
. That means
"CREATE MATERIALIZED VIEW MV_TEST BUILD IMMEDIATE REFRESH FAST ON COMMIT AS...." does a fast-refresh
"CREATE MATERIALIZED VIEW MV_TEST BUILD IMMEDIATE REFRESH ON COMMIT AS...." does a on-commit