DISTINCT INNER JOIN慢

Question

我已經寫了下面的PostgreSQL查詢，它的工作原理應該如此。但是，它似乎非常緩慢，有時需要10秒才能返回結果。我確信在我的發言中有一些事情導致這種情況變得緩慢。

任何人都可以幫助確定爲什麼這個查詢是慢？

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')) 

我取代NOT IN具有以下：

 
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))))

Answer 1

NOT EXISTS是一個很好的選擇：

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')) 

的EXPLAIN ANALYZE結果。幾乎總是比NOT IN更好。 More details here. 我簡單查詢了一下（這看起來不錯，通常情況下）：

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, ???

什麼似乎缺少的是ORDER BYwhich should accompany your DISTINCT ON。增加更多ORDER BY項目代替???。如果有重複選擇，您可能想要定義哪個要選擇。

Numeric literals不需要單引號和boolean值應該這樣編碼。
您可能需要重新訪問chapter about data types。