problem in filesort.cc

MySQL & MariaDB Other MySQL® Questions
1

i am reading code of sort,here is the code that i do not understand very much,my question is what kind of sql is “quick select” ?

188 if (select && select->quick)
189 {
190 status_var_increment(thd-status_var.filesort_range_count);
191 }
192 else
193 {
194 status_var_increment(thd->status_var.filesort_scan_count);
195 }