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

revno: 4151

committer: Olav Sandstaa <olav.sandstaa@oracle.com>

branch nick: loose-scan-perf-5.6

timestamp: Thu 2012-08-23 09:45:33 +0200

message:

  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

  calculting the cost estimates becomes almost identical to the cost of using

  normal index scan. Differences in use of integer versus floating point aritmetic

  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.