When a derived table which has distinct values and BLOB fields is
materialized, an index is created over all columns to ensure only
distinct values are placed to the result.
This index is created in a special mode HA_UNIQUE_HASH to support BLOBs.
Later in `best_access_path` the optimizer may incorrectly choose this
index to retrieve values from the derived table, although such type
of index cannot be used for data retrieval.

This commit excludes HA_UNIQUE_HASH indexes from consideration
in `best_access_path`