Bug#111210 fix bug optimizer reverse index scan give incorrect result

[srmppn]

Description

This pull request fixes Bug#119198, which occurs when a reverse index range scan returns incorrect results.

During SQL query optimization, the used_key_parts may need to be extended — for example, when a query requires ordering by both a column and its primary key. However, the optimizer did not properly check for this condition, causing m_using_extended_key_parts in the reverse_index_range_scan to remain false. As a result, the logic incorrectly falls into the eqrange_all_keyparts section instead, leading to wrong results:

if (eqrange_all_keyparts) { result = table()->file->ha_index_read_map( table()->record[0], last_range->max_key, last_range->max_keypart_map, HA_READ_KEY_EXACT); } 

Proposed solution

Assign used_key_parts variable to retrieve possible extended key parts — similar to the approach used in test_if_skip_sort_order method. Additionally, I check for the maximum value using std::max to ensure we use extended key parts.