Answer by Frank Heikens for PostgreSQL: reuse of complex intermediate result in same query

You need an index on test1_id in table test2, that will change things.

Seq Scan on test1 t1  (cost=0.00..301450.63 rows=8761 width=12) (actual time=0.108..229.859 rows=8761 loops=1)  SubPlan 1    ->  Aggregate  (cost=11.45..11.46 rows=1 width=4) (actual time=0.008..0.008 rows=1 loops=8761)          ->  Bitmap Heap Scan on test2 t2  (cost=3.27..11.45 rows=1 width=4) (actual time=0.007..0.007 rows=0 loops=8761)                Recheck Cond: (test1_id = t1.id)                Filter: ((category)::text = 'A'::text)                ->  Bitmap Index Scan on idx_id  (cost=0.00..3.27 rows=3 width=0) (actual time=0.003..0.003 rows=3 loops=8761)                      Index Cond: (test1_id = t1.id)  SubPlan 2    ->  Aggregate  (cost=11.45..11.46 rows=1 width=4) (actual time=0.007..0.008 rows=1 loops=8761)          ->  Bitmap Heap Scan on test2 t2  (cost=3.27..11.45 rows=1 width=4) (actual time=0.006..0.006 rows=0 loops=8761)                Recheck Cond: (test1_id = t1.id)                Filter: ((category)::text = 'B'::text)                ->  Bitmap Index Scan on idx_id  (cost=0.00..3.27 rows=3 width=0) (actual time=0.003..0.003 rows=3 loops=8761)                      Index Cond: (test1_id = t1.id)  SubPlan 3    ->  Aggregate  (cost=11.46..11.47 rows=1 width=8) (actual time=0.008..0.008 rows=1 loops=8761)          ->  Bitmap Heap Scan on test2 t2  (cost=3.27..11.45 rows=1 width=8) (actual time=0.006..0.006 rows=0 loops=8761)                Recheck Cond: (test1_id = t1.id)                Filter: ((category)::text = 'C'::text)                ->  Bitmap Index Scan on idx_id  (cost=0.00..3.27 rows=3 width=0) (actual time=0.003..0.003 rows=3 loops=8761)                      Index Cond: (test1_id = t1.id)Total runtime: 232.419 ms