Fixed the problem of mdev-5947.
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.
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