This bug is a regression bug. The regression was introduced by
the patch for mdev-3851, that tried to weaken the condition when
a ref access with an extended key can be converted to an eq_ref
access. The patch incorrectly formed this condition. As a result,
while improving performance for some queries, the patch caused
worse performance for another queries.
When inserting a record with update on duplicate keys the server calls
the ha_index_read_idx_map handler function to look for the record
that violates unique key constraints. The third parameter of this call
should mark only the base components of the index where the server is
searched for the record. Possible hidden components of the primary key
are to be unmarked.
When working on MWL#247 I forgot to adjust the function create_hj_key_for_table()
that created a key definition for hash join keys. The modified function must
set the values of the fields ext_key_parts, ext_key_flags, ext_key_part_map
added to the key definition structure in MWL#247.
The fields ext_key_flags and ext_key_part_map must be initialized for any
key, even for a MyISAM key that never is considered by the optimizer as one
extended by hidden components.
The patch for MWL #247 forgot to initialize the TABLE::ext_key_parts and
TABLE::ext_key_flags of the temporary tables by a query. This could cause
crashes for queries the execution of which needed creation of temporary
tables.
