why 1 reducer on simple join?

[Koert Kuipers]

I am running a basic join of 2 tables and it will only run with 1 reducer.
why is that? i tried to set the number of reducers and it didn't work. hive
just ignored it.

create table z as select x.* from table1 x join table2 y where (
x.col1 = y.col1 and
x.col2 = y.col2 and
x.col3 = y.col3 and
x.col4 = y.col4 and
x.col5 = y.col5
);

both tables are backed by multiple files / blocks / chunks