Back-ported from the mysql 5.6 code line the patch with
the following comment:

  Fix for Bug#11757108 CHANGE IN EXECUTION PLAN FOR COUNT_DISTINCT_GROUP_ON_KEY
                       CAUSES PEFORMANCE REGRESSION

  The cause for the performance regression is that the access strategy for the
  GROUP BY query is changed form using "index scan" in mysql-5.1 to use "loose
  index scan" in mysql-5.5. The index used for group by is unique and thus each
  "loose scan" group will only contain one record. Since loose scan needs to
  re-position on each "loose scan" group this query will do a re-position for
  each index entry. Compared to just reading the next index entry as a normal
  index scan does, the use of loose scan for this query becomes more expensive.

  The cause for selecting to use loose scan for this query is that in the current
  code when the size of the "loose scan" group is one, the formula for
  calculating the cost estimates becomes almost identical to the cost of using
  normal index scan. Differences in use of integer versus floating point arithmetic
  can cause one or the other access strategy to be selected.

  The main issue with the formula for estimating the cost of using loose scan is
  that it does not take into account that it is more costly to do a re-position
  for each "loose scan" group compared to just reading the next index entry.
  Both index scan and loose scan estimates the cpu cost as:

    "number of entries needed too read/scan" * ROW_EVALUATE_COST

  The results from testing with the query in this bug indicates that the real
  cost for doing re-position four to eight times higher than just reading the
  next index entry. Thus, the cpu cost estimate for loose scan should be increased.
  To account for the extra work to re-position in the index we increase the
  cost for loose index scan to include the cost of navigating the index.
  This is modelled as a function of the height of the b-tree:

    navigation cost= ceil(log(records in table)/log(indexes per block))
                   * ROWID_COMPARE_COST;

  This will avoid loose index scan being used for indexes where the "loose scan"
  group contains very few index entries.