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些内容会导致这种情况变慢。
任何人都可以帮助确定此查询为何缓慢吗？
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') )
我将NOT IN替换为以下内容：
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))))
答案 0 :(得分：1)
NOT EXISTS是一个很好的选择。几乎总是比NOT IN好。 More details here.
我简化了你的查询（通常看起来很好）：
SELECT DISTINCT ON (c.class_id, a.school_date)
c.class_id, c.class_name, c.grade_id
,g.linked_calendar, aa.calendar_id
,a.school_date, a.minutes
,t.staff_id, s.first_name, s.last_name
FROM school_classes c
JOIN teacher_join_classes_subjects t USING (class_id)
JOIN staff s USING (staff_id)
JOIN school_gradelevels g ON g.id = c.grade_id
JOIN attendance_calendars aa ON aa.title = g.linked_calendar
JOIN attendance_calendar a ON a.calendar_id = aa.calendar_id
WHERE t.syear = 2013
AND s.syear = 2013
AND aa.syear = 2013
AND t.does_attendance = 'Y' -- looks like it should be boolean!
AND t.subject_id IS NULL
AND a.school_date < CURRENT_DATE
AND NOT EXISTS (
SELECT 1
FROM attendance_completed x
WHERE x.class_id = c.class_id
AND x.school_date = a.school_date
AND (x.period_id = 101 AND a.minutes >= 151 OR -- actually numbers?
x.period_id = 95 AND a.minutes = 150)
)
ORDER BY c.class_id, a.school_date, ???似乎缺少的是ORDER BY which should accompany your DISTINCT ON。添加更多ORDER BY项代替???。如果有重复项可供选择，您可能需要定义选择哪个。
Numeric literals不需要单引号，boolean值应按此编码
您可能想要重新访问chapter about data types。