I just added it as you said, but I am getting same plan.
Sort (cost=62842.16..62846.91 rows=1897 width=35) (actual time=1845.831..1845.950 rows=1229 loops=1)
Sort Method: quicksort Memory: 145kB
-> HashAggregate (cost=62710.42..62738.88 rows=1897 width=35) (actual time=1844.178..1845.060 rows=1229 loops=1)
-> Hash Right Join (cost=159.68..45382.09 rows=364807 width=35) (actual time=1.534..618.717 rows=365784 loops=1)
Filter: (workitems.deleted_at IS NULL)
Rows Removed by Filter: 257457
-> Seq Scan on workitems (cost=0.00..36653.75 rows=623175 width=43) (actual time=0.047..213.842 rows=623175 loops=1)
-> Hash (cost=135.97..135.97 rows=1897 width=16) (actual time=1.478..1.478 rows=1897 loops=1)
Buckets: 2048 Batches: 1 Memory Usage: 105kB
-> Seq Scan on projects (cost=0.00..135.97 rows=1897 width=16) (actual time=0.006..0.914 rows=1897 loops=1)
Planning time: 0.498 ms
Execution time: 1846.100 ms
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Indexes:
"workitems_pkey" PRIMARY KEY, btree (id)
"index_workitems_on_company_id" btree (company_id)
"index_workitems_on_deleted_at" btree (deleted_at)
"index_workitems_on_parent_workitem_id" btree (parent_workitem_id)
"index_workitems_on_project_id" btree (project_id)
"index_workitems_on_standard_workitem_id" btree (standard_workitem_id)
"index_workitems_on_workitem_category_id" btree (workitem_category_id)
"patrial_index_workitems_200_1" btree (project_id) WHERE deleted_at IS NULL
CREATE INDEX ON workitems(project_id) WHERE deleted_at is null
