ANALYSIS

This is regression caused due to worklog 6742 which
implemented ha_innobase::records() which always
uses clustered index to get the row count. Previously
optimizer chose secondary index which was smaller in
size of clustered index to scan for rows and resulted in
a quicker scan.

FIX

After discussion it was decided to remove this feature in 5.7.

[#rb14040 Approved by Kevin and Oystein ]