Filesort with Small LIMIT Optimization
n is sufficiently small, the optimizer will use a priority queue for sorting. Before the optimization's porting to MariaDB 10.0, the alternative was, roughly speaking, to sort the entire output and then pick only first
Optimization Visibility in MariaDB
There are two ways to check whether filesort has used a priority queue.
The first way is to check the Sort_priority_queue_sorts status variable. It shows the number of times that sorting was done through a priority queue. (The total number of times sorting was done is a sum Sort_range and Sort_scan).
Slow Query Log
# Time: 140714 18:30:39 # User@Host: root[root] @ localhost  # Thread_id: 3 Schema: test QC_hit: No # Query_time: 0.053857 Lock_time: 0.000188 Rows_sent: 11 Rows_examined: 100011 # Full_scan: Yes Full_join: No Tmp_table: No Tmp_table_on_disk: No # Filesort: Yes Filesort_on_disk: No Merge_passes: 0 Priority_queue: Yes SET timestamp=1405348239;SET timestamp=1405348239; select * from t1 where col1 between 10 and 20 order by col2 limit 100;
Note the "Priority_queue: Yes" on the last comment line. (
pt-query-digest is able to parse slow query logs with the Priority_queue field)
EXPLAIN, it will give no indication whether filesort uses priority queue or the generic quicksort and merge algorithm.
Using filesort will be shown in both cases, by both MariaDB and MySQL.