USE_INDEX_EXTENSIONS=OFF

Issue:
------
CREATE TABLE testtable (
 fielda INTEGER NOT NULL,
 fieldb INTEGER NOT NULL,
 fieldc INTEGER NOT NULL,
 PRIMARY KEY (fielda,fieldb),
 KEY fieldb_idx (fieldb)
) ENGINE=Innodb;

SELECT DISTINCT fieldb
FROM testtable
WHERE fielda IN (...) AND fieldb IN (...);

With index extensions turned off, fielda shouldn't be used
as part of fieldb_idx. So when the query tries to use loose
index scan on fieldb_idx, the corresponding range tree for
the lookup doesn't contain fielda. Since this approach
involves a lookup of the index fieldb_idx, we get a wrong
result.

Solution:
---------
Loose index scan is the wrong choice for this query given
that fielda is not covered by fieldb_idx when
index_extensions is turned off.

Each field has a flag, part_of_key_not_clustered which
tracks the indexes that cover the field. When index
extensions is turned off, this flag can be used to track
whether a particular field is part of the secondary index
or is part of the primary key that has been appended by the
storage engine..