DISTINCT INNER JOIN慢

我编写了以下PostgreSQL查询，它可以正常工作 . 但是，它看起来非常慢，有时需要10秒才能返回结果 . 我确信我的陈述中有一些东西导致这种情况变慢 .

任何人都可以帮助确定为什么这个查询很慢？

SELECT DISTINCT ON (school_classes.class_id,attendance_calendar.school_date)
  school_classes.class_id, school_classes.class_name, school_classes.grade_id
, school_gradelevels.linked_calendar, attendance_calendars.calendar_id
, attendance_calendar.school_date, attendance_calendar.minutes
, teacher_join_classes_subjects.staff_id, staff.first_name, staff.last_name  

FROM school_classes 
INNER JOIN school_gradelevels ON school_gradelevels.id=school_classes.grade_id 
INNER JOIN teacher_join_classes_subjects ON teacher_join_classes_subjects.class_id=school_classes.class_id 
INNER JOIN staff ON staff.staff_id=teacher_join_classes_subjects.staff_id 
INNER JOIN attendance_calendars ON attendance_calendars.title=school_gradelevels.linked_calendar 
INNER JOIN attendance_calendar ON attendance_calendar.calendar_id=attendance_calendars.calendar_id 

WHERE teacher_join_classes_subjects.syear='2013' 
AND staff.syear='2013' 
AND attendance_calendars.syear='2013' 
AND teacher_join_classes_subjects.does_attendance='Y' 
AND teacher_join_classes_subjects.subject_id IS NULL 
AND attendance_calendar.school_date<CURRENT_DATE 

AND attendance_calendar.school_date NOT IN (

SELECT com.school_date FROM attendance_completed com
WHERE  com.class_id=school_classes.class_id
AND   (com.period_id='101' AND attendance_calendar.minutes>='151' OR
       com.period_id='95'  AND attendance_calendar.minutes='150') )

我用以下内容替换了 NOT IN ：

AND NOT EXISTS (
    SELECT com.school_date
    FROM attendance_completed com
    WHERE com.class_id=school_classes.class_id
    AND com.school_date=attendance_calendar.school_date
    AND (com.period_id='101' AND attendance_calendar.minutes>='151' OR
         com.period_id='95'  AND attendance_calendar.minutes='150') )

EXPLAIN ANALYZE的结果：

Unique  (cost=2998.39..2998.41 rows=3 width=85) (actual time=10751.111..10751.118 rows=1 loops=1)
  ->  Sort  (cost=2998.39..2998.40 rows=3 width=85) (actual time=10751.110..10751.110 rows=2 loops=1)
        Sort Key: school_classes.class_id, attendance_calendar.school_date
        Sort Method:  quicksort  Memory: 25kB
        ->  Hash Join  (cost=2.03..2998.37 rows=3 width=85) (actual time=6409.471..10751.045 rows=2 loops=1)
              Hash Cond: ((teacher_join_classes_subjects.class_id = school_classes.class_id) AND (school_gradelevels.id = school_classes.grade_id))
              Join Filter: (NOT (SubPlan 1))
              ->  Nested Loop  (cost=0.00..120.69 rows=94 width=81) (actual time=2.468..1187.397 rows=26460 loops=1)
                    Join Filter: (attendance_calendars.calendar_id = attendance_calendar.calendar_id)
                    ->  Nested Loop  (cost=0.00..42.13 rows=1 width=70) (actual time=0.087..3.247 rows=735 loops=1)
                          Join Filter: ((attendance_calendars.title)::text = (school_gradelevels.linked_calendar)::text)
                          ->  Nested Loop  (cost=0.00..40.80 rows=1 width=277) (actual time=0.077..1.005 rows=245 loops=1)
                                ->  Nested Loop  (cost=0.00..39.61 rows=1 width=27) (actual time=0.064..0.572 rows=49 loops=1)
                                      ->  Seq Scan on teacher_join_classes_subjects  (cost=0.00..10.48 rows=4 width=14) (actual time=0.022..0.143 rows=49 loops=1)
                                            Filter: ((subject_id IS NULL) AND (syear = 2013::numeric) AND ((does_attendance)::text = 'Y'::text))
                                      ->  Index Scan using staff_pkey on staff  (cost=0.00..7.27 rows=1 width=20) (actual time=0.006..0.007 rows=1 loops=49)
                                            Index Cond: (staff.staff_id = teacher_join_classes_subjects.staff_id)
                                            Filter: (staff.syear = 2013::numeric)
                                ->  Seq Scan on attendance_calendars  (cost=0.00..1.18 rows=1 width=250) (actual time=0.003..0.006 rows=5 loops=49)
                                      Filter: (attendance_calendars.syear = 2013::numeric)
                          ->  Seq Scan on school_gradelevels  (cost=0.00..1.15 rows=15 width=11) (actual time=0.001..0.005 rows=15 loops=245)
                    ->  Seq Scan on attendance_calendar  (cost=0.00..55.26 rows=1864 width=18) (actual time=0.003..1.129 rows=1824 loops=735)
                          Filter: (attendance_calendar.school_date   Hash  (cost=1.41..1.41 rows=41 width=18) (actual time=0.040..0.040 rows=41 loops=1)
                    ->  Seq Scan on school_classes  (cost=0.00..1.41 rows=41 width=18) (actual time=0.006..0.015 rows=41 loops=1)
              SubPlan 1
                ->  Seq Scan on attendance_completed com  (cost=0.00..958.28 rows=5 width=4) (actual time=0.228..5.411 rows=17 loops=1764)
                      Filter: ((class_id = $0) AND (((period_id = 101::numeric) AND ($1 >= 151::numeric)) OR ((period_id = 95::numeric) AND ($1 = 150::numeric))))