diff options
Diffstat (limited to 'sql/filesort.cc')
| -rw-r--r-- | sql/filesort.cc | 13 |
1 files changed, 6 insertions, 7 deletions
diff --git a/sql/filesort.cc b/sql/filesort.cc index 07194cb2e4f..988df356348 100644 --- a/sql/filesort.cc +++ b/sql/filesort.cc @@ -1584,6 +1584,10 @@ static bool check_if_pq_applicable(Sort_param *param, // Can we fit all the keys in memory? if (param->max_keys_per_buffer < num_available_keys) { + /* + Get cost of merge sort. Note that this does not include + scanning the table and comparing the rows with the where clause! + */ const double sort_merge_cost= get_merge_many_buffs_cost_fast(num_rows, num_available_keys, @@ -1591,17 +1595,12 @@ static bool check_if_pq_applicable(Sort_param *param, /* PQ has cost: (insert + qsort) * log(queue size) / TIME_FOR_COMPARE_ROWID + - cost of file lookup afterwards. - The lookup cost is a bit pessimistic: we take scan_time and assume - that on average we find the row after scanning half of the file. - A better estimate would be lookup cost, but note that we are doing - random lookups here, rather than sequential scan. + cost of rowid lookup of the original row to find the addon fields. */ const double pq_cpu_cost= (PQ_slowness * num_rows + param->max_keys_per_buffer) * log((double) param->max_keys_per_buffer) / TIME_FOR_COMPARE_ROWID; - const double pq_io_cost= - param->max_rows * table->file->scan_time() / 2.0; + const double pq_io_cost= table->file->ha_read_with_rowid(param->max_rows); const double pq_cost= pq_cpu_cost + pq_io_cost; if (sort_merge_cost < pq_cost) |