In the function create_key_parts_for_pseudo_indexes()
the key part structures of pseudo-indexes created for
BLOB fields were set incorrectly.
Also the key parts for long fields must be 'truncated'
up to the maximum length acceptable for key parts.
- Fix the crash by making get_column_range_cardinality()
to handle the special case where Column_stats objects
is an all-zeros object (the question of what is the point
of having Field::read_stats point to such object remains a
mystery)
- Added a few comments. Learning the code still.
