Analysis:
The bug is a result of an incomplete fix for bug lp:869036.
That fix didn't take into account that there may be a case
when ther are no NULLs in the materialized subquery, however
all columns without NULLs may not be grouped in the only
non-null index. This is the case when the left subquery expression
has nullable columns.

Solution:
The patch handles two missing sub-cases of the case when there are
no value (non-null matches) for any outer expression, and there are
both NULLs and non-NUll values in the outer reference.
a) If the materialized subquery contains no NULLs there cannot be a
   partial match, because there are no NULLs in those columns where
   the outer reference has no NULLs.
b) If the materialized subquery contains NULLs, but there exists a
   column, such that its corresponding outer expression has no NULL,
   and this column also has no NULL. Then there cannot be a partial
   match either.