On the topic view route we query for reviewables of each post in the stream,
using a query that filters on two unindexed columns. This results in a Parallel Seq Scan
over all rows, which can take quite some time (~20ms was seen) on forums with lots of flags
After index is added PostgreSQL planner opts for a simple Index Scan and runs in sub 1ms.
Before:
```
QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Finalize GroupAggregate (cost=11401.08..11404.87 rows=20 width=28) (actual time=19.209..19.209 rows=1 loops=1)
Group Key: r.target_id
-> Gather Merge (cost=11401.08..11404.41 rows=26 width=28) (actual time=19.202..20.419 rows=1 loops=1)
Workers Planned: 2
Workers Launched: 2
-> Partial GroupAggregate (cost=10401.06..10401.38 rows=13 width=28) (actual time=16.958..16.958 rows=0 loops=3)
Group Key: r.target_id
-> Sort (cost=10401.06..10401.09 rows=13 width=16) (actual time=16.956..16.956 rows=0 loops=3)
Sort Key: r.target_id
Sort Method: quicksort Memory: 25kB
Worker 0: Sort Method: quicksort Memory: 25kB
Worker 1: Sort Method: quicksort Memory: 25kB
-> Nested Loop (cost=0.42..10400.82 rows=13 width=16) (actual time=15.894..16.938 rows=0 loops=3)
-> Parallel Seq Scan on reviewables r (cost=0.00..10302.47 rows=8 width=12) (actual time=15.882..16.927 rows=0 loops=3)
Filter: (((target_type)::text = 'Post'::text) AND (target_id = ANY ('{7565483,7565563,7565566,7565567,7565568,7565569,7565579,7565580,7565583,7565586,7565588,7565589,7565601,7565602,7565603,7565613,7565620,7565623,7565624,7565626}'::integer[])))
Rows Removed by Filter: 49183
-> Index Scan using index_reviewable_scores_on_reviewable_id on reviewable_scores s (cost=0.42..12.27 rows=2 width=8) (actual time=0.029..0.030 rows=1 loops=1)
Index Cond: (reviewable_id = r.id)
Planning Time: 0.318 ms
Execution Time: 20.470 ms
```
After:
```
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
GroupAggregate (cost=0.84..342.54 rows=20 width=28) (actual time=0.038..0.038 rows=1 loops=1)
Group Key: r.target_id
-> Nested Loop (cost=0.84..341.95 rows=31 width=16) (actual time=0.020..0.033 rows=1 loops=1)
-> Index Scan using index_reviewables_on_target_id on reviewables r (cost=0.42..96.07 rows=20 width=12) (actual time=0.013..0.026 rows=1 loops=1)
Index Cond: (target_id = ANY ('{7565483,7565563,7565566,7565567,7565568,7565569,7565579,7565580,7565583,7565586,7565588,7565589,7565601,7565602,7565603,7565613,7565620,7565623,7565624,7565626}'::integer[]))
-> Index Scan using index_reviewable_scores_on_reviewable_id on reviewable_scores s (cost=0.42..12.27 rows=2 width=8) (actual time=0.005..0.005 rows=1 loops=1)
Index Cond: (reviewable_id = r.id)
Planning Time: 0.253 ms
Execution Time: 0.067 ms
```
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