#	exclusive	inclusive	rows x	rows	loops	node
1.	0.053	150,900.484	↓ 3.0	3	1	→ HashAggregate (cost=2,329.88..2,329.89 rows=1 width=16) (actual time=150,900.483..150,900.484 rows=3 loops=1) Group Key: c.id, g.id
2.	6,975.498	150,900.431	↓ 228.0	228	1	→ Nested Loop (cost=506.00..2,329.88 rows=1 width=16) (actual time=121,413.363..150,900.431 rows=228 loops=1)
3.	2,375.910	18,203.813	↓ 1,381,550.8	17,960,160	1	→ Nested Loop (cost=505.58..2,289.95 rows=13 width=24) (actual time=5.858..18,203.813 rows=17,960,160 loops=1)
4.	1,136.824	9,939.291	↓ 1,472,153.0	1,472,153	1	→ Nested Loop (cost=505.29..2,288.68 rows=1 width=40) (actual time=5.838..9,939.291 rows=1,472,153 loops=1) Join Filter: (cm.course = context.instanceid)
5.	182.436	4,386.008	↓ 1,472,153.0	1,472,153	1	→ Nested Loop (cost=504.87..2,287.93 rows=1 width=64) (actual time=5.814..4,386.008 rows=1,472,153 loops=1) Join Filter: (cm.course = c.id)
6.	250.079	1,259.266	↓ 1,472,153.0	1,472,153	1	→ Nested Loop (cost=504.58..2,287.55 rows=1 width=56) (actual time=5.799..1,259.266 rows=1,472,153 loops=1)
7.	12.527	186.101	↓ 45,727.0	45,727	1	→ Nested Loop (cost=504.29..2,286.06 rows=1 width=40) (actual time=5.787..186.101 rows=45,727 loops=1)
8.	2.588	103.559	↓ 3,685.0	3,685	1	→ Nested Loop (cost=504.00..2,285.14 rows=1 width=40) (actual time=5.750..103.559 rows=3,685 loops=1)
9.	5.576	30.956	↓ 3,685.0	3,685	1	→ Hash Join (cost=503.58..2,280.85 rows=1 width=40) (actual time=5.723..30.956 rows=3,685 loops=1) Hash Cond: ((cm.course = q.course) AND (cm.instance = q.id))
10.	19.971	20.808	↑ 1.0	3,686	1	→ Bitmap Heap Scan on mdl_course_modules cm (cost=73.46..1,822.49 rows=3,763 width=24) (actual time=1.094..20.808 rows=3,686 loops=1) Recheck Cond: (module = 18) Heap Blocks: exact=1,186
11.	0.837	0.837	↑ 1.0	3,686	1	→ Bitmap Index Scan on mdl_courmodu_mod_ix (cost=0.00..72.52 rows=3,763 width=0) (actual time=0.837..0.837 rows=3,686 loops=1) Index Cond: (module = 18)
12.	2.149	4.572	↑ 1.0	3,685	1	→ Hash (cost=374.85..374.85 rows=3,685 width=16) (actual time=4.572..4.572 rows=3,685 loops=1) Buckets: 4,096 Batches: 1 Memory Usage: 205kB
13.	2.423	2.423	↑ 1.0	3,685	1	→ Seq Scan on mdl_quiz q (cost=0.00..374.85 rows=3,685 width=16) (actual time=0.006..2.423 rows=3,685 loops=1)
14.	70.015	70.015	↑ 1.0	1	3,685	→ Index Scan using mdl_cont_conins_uix on mdl_context contextm (cost=0.42..4.28 rows=1 width=16) (actual time=0.019..0.019 rows=1 loops=3,685) Index Cond: ((contextlevel = 70) AND (instanceid = cm.id))
15.	70.015	70.015	↑ 1.8	12	3,685	→ Index Scan using mdl_quesusag_con_ix on mdl_question_usages qu (cost=0.29..0.70 rows=22 width=16) (actual time=0.004..0.019 rows=12 loops=3,685) Index Cond: (contextid = contextm.id)
16.	823.086	823.086	↓ 4.0	32	45,727	→ Index Scan using mdl_grou_cou_ix on mdl_groups g (cost=0.29..1.41 rows=8 width=16) (actual time=0.003..0.018 rows=32 loops=45,727) Index Cond: (courseid = cm.course)
17.	2,944.306	2,944.306	↑ 1.0	1	1,472,153	→ Index Only Scan using mdl_cour_id_pk on mdl_course c (cost=0.29..0.37 rows=1 width=8) (actual time=0.001..0.002 rows=1 loops=1,472,153) Index Cond: (id = g.courseid) Heap Fetches: 1,472,153
18.	4,416.459	4,416.459	↑ 1.0	1	1,472,153	→ Index Scan using mdl_cont_ins_ix on mdl_context context (cost=0.42..0.75 rows=1 width=8) (actual time=0.003..0.003 rows=1 loops=1,472,153) Index Cond: (instanceid = g.courseid) Filter: (contextlevel = 50) Rows Removed by Filter: 3
19.	5,888.612	5,888.612	↑ 1.1	12	1,472,153	→ Index Only Scan using mdl_quizslot_qui_ix on mdl_quiz_slots qs (cost=0.29..1.13 rows=13 width=8) (actual time=0.002..0.004 rows=12 loops=1,472,153) Index Cond: (quizid = q.id) Heap Fetches: 17,960,160
20.	125,721.120	125,721.120	↓ 0.0	0	17,960,160	→ Index Scan using mdl_quesatte_que2_ix on mdl_question_attempts att (cost=0.42..3.06 rows=1 width=8) (actual time=0.007..0.007 rows=0 loops=17,960,160) Index Cond: (questionusageid = qu.id) Filter: (questionid = ANY ('{686662,686663}'::bigint[])) Rows Removed by Filter: 18

Planning time	:	36.273 ms
Execution time	:	150,900.737 ms

"HashAggregate  (cost=2329.88..2329.89 rows=1 width=16) (actual time=150900.483..150900.484 rows=3 loops=1)"
"  Group Key: c.id, g.id"
"  ->  Nested Loop  (cost=506.00..2329.88 rows=1 width=16) (actual time=121413.363..150900.431 rows=228 loops=1)"
"        ->  Nested Loop  (cost=505.58..2289.95 rows=13 width=24) (actual time=5.858..18203.813 rows=17960160 loops=1)"
"              ->  Nested Loop  (cost=505.29..2288.68 rows=1 width=40) (actual time=5.838..9939.291 rows=1472153 loops=1)"
"                    Join Filter: (cm.course = context.instanceid)"
"                    ->  Nested Loop  (cost=504.87..2287.93 rows=1 width=64) (actual time=5.814..4386.008 rows=1472153 loops=1)"
"                          Join Filter: (cm.course = c.id)"
"                          ->  Nested Loop  (cost=504.58..2287.55 rows=1 width=56) (actual time=5.799..1259.266 rows=1472153 loops=1)"
"                                ->  Nested Loop  (cost=504.29..2286.06 rows=1 width=40) (actual time=5.787..186.101 rows=45727 loops=1)"
"                                      ->  Nested Loop  (cost=504.00..2285.14 rows=1 width=40) (actual time=5.750..103.559 rows=3685 loops=1)"
"                                            ->  Hash Join  (cost=503.58..2280.85 rows=1 width=40) (actual time=5.723..30.956 rows=3685 loops=1)"
"                                                  Hash Cond: ((cm.course = q.course) AND (cm.instance = q.id))"
"                                                  ->  Bitmap Heap Scan on mdl_course_modules cm  (cost=73.46..1822.49 rows=3763 width=24) (actual time=1.094..20.808 rows=3686 loops=1)"
"                                                        Recheck Cond: (module = 18)"
"                                                        Heap Blocks: exact=1186"
"                                                        ->  Bitmap Index Scan on mdl_courmodu_mod_ix  (cost=0.00..72.52 rows=3763 width=0) (actual time=0.837..0.837 rows=3686 loops=1)"
"                                                              Index Cond: (module = 18)"
"                                                  ->  Hash  (cost=374.85..374.85 rows=3685 width=16) (actual time=4.572..4.572 rows=3685 loops=1)"
"                                                        Buckets: 4096  Batches: 1  Memory Usage: 205kB"
"                                                        ->  Seq Scan on mdl_quiz q  (cost=0.00..374.85 rows=3685 width=16) (actual time=0.006..2.423 rows=3685 loops=1)"
"                                            ->  Index Scan using mdl_cont_conins_uix on mdl_context contextm  (cost=0.42..4.28 rows=1 width=16) (actual time=0.019..0.019 rows=1 loops=3685)"
"                                                  Index Cond: ((contextlevel = 70) AND (instanceid = cm.id))"
"                                      ->  Index Scan using mdl_quesusag_con_ix on mdl_question_usages qu  (cost=0.29..0.70 rows=22 width=16) (actual time=0.004..0.019 rows=12 loops=3685)"
"                                            Index Cond: (contextid = contextm.id)"
"                                ->  Index Scan using mdl_grou_cou_ix on mdl_groups g  (cost=0.29..1.41 rows=8 width=16) (actual time=0.003..0.018 rows=32 loops=45727)"
"                                      Index Cond: (courseid = cm.course)"
"                          ->  Index Only Scan using mdl_cour_id_pk on mdl_course c  (cost=0.29..0.37 rows=1 width=8) (actual time=0.001..0.002 rows=1 loops=1472153)"
"                                Index Cond: (id = g.courseid)"
"                                Heap Fetches: 1472153"
"                    ->  Index Scan using mdl_cont_ins_ix on mdl_context context  (cost=0.42..0.75 rows=1 width=8) (actual time=0.003..0.003 rows=1 loops=1472153)"
"                          Index Cond: (instanceid = g.courseid)"
"                          Filter: (contextlevel = 50)"
"                          Rows Removed by Filter: 3"
"              ->  Index Only Scan using mdl_quizslot_qui_ix on mdl_quiz_slots qs  (cost=0.29..1.13 rows=13 width=8) (actual time=0.002..0.004 rows=12 loops=1472153)"
"                    Index Cond: (quizid = q.id)"
"                    Heap Fetches: 17960160"
"        ->  Index Scan using mdl_quesatte_que2_ix on mdl_question_attempts att  (cost=0.42..3.06 rows=1 width=8) (actual time=0.007..0.007 rows=0 loops=17960160)"
"              Index Cond: (questionusageid = qu.id)"
"              Filter: (questionid = ANY ('{686662,686663}'::bigint[]))"
"              Rows Removed by Filter: 18"
"Planning time: 36.273 ms"
"Execution time: 150900.737 ms"

Per node type stats

node type	count	sum of times	% of query
Bitmap Heap Scan	1	19.971 ms	0.0 %
Bitmap Index Scan	1	0.837 ms	0.0 %
Hash	1	2.149 ms	0.0 %
Hash Join	1	5.576 ms	0.0 %
HashAggregate	1	0.053 ms	0.0 %
Index Only Scan	2	8,832.918 ms	5.9 %
Index Scan	5	131,100.695 ms	86.9 %
Nested Loop	7	10,935.862 ms	7.2 %
Seq Scan	1	2.423 ms	0.0 %

Per table stats

Table name	Scan count	Total time	% of query
scan type	count	sum of times	% of table
mdl_context	2	4,486.474 ms	3.0 %
Index Scan	2	4,486.474 ms	100.0 %
mdl_course	1	2,944.306 ms	2.0 %
Index Only Scan	1	2,944.306 ms	100.0 %
mdl_course_modules	1	19.971 ms	0.0 %
Bitmap Heap Scan	1	19.971 ms	100.0 %
mdl_groups	1	823.086 ms	0.5 %
Index Scan	1	823.086 ms	100.0 %
mdl_question_attempts	1	125,721.120 ms	83.3 %
Index Scan	1	125,721.120 ms	100.0 %
mdl_question_usages	1	70.015 ms	0.0 %
Index Scan	1	70.015 ms	100.0 %
mdl_quiz	1	2.423 ms	0.0 %
Seq Scan	1	2.423 ms	100.0 %
mdl_quiz_slots	1	5,888.612 ms	3.9 %
Index Only Scan	1	5,888.612 ms	100.0 %

explain.depesz.com

PostgreSQL's explain analyze made readable

Result: yBIj

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Visible columns:

Per node type stats

Per table stats