plan with result cache is very slow when work_mem is not enough

Hi

I am testing new features of Postgres 14, and now I am trying to check the
result cache. Unfortunately on my test data, the result is not too good.
the behaviour is very non linear. Is it expected?

create table t1(a int, t2_id int);
insert into t1 select random() * 100000, random() * 100000 from
generate_series(1,1000000);
create table t2(b int, id int);
insert into t2 select random() * 100000, random() * 100000 from
generate_series(1,1000000);
create index on t2(id);

vacuum analyze t1, t2;

when work_mem is 40MB

QUERY PLAN

-------------------------------------------------------------------------------------------------------------------------------------------

Nested Loop (cost=4.65..472639.79 rows=1000000 width=16) (actual
time=0.041..1078.882 rows=1000000 loops=1)

-> Seq Scan on t1 (cost=0.00..14425.00 rows=1000000 width=8) (actual
time=0.010..60.212 rows=1000000 loops=1)

-> Result Cache (cost=4.65..4.67 rows=1 width=8) (actual
time=0.001..0.001 rows=1 loops=1000000)

Cache Key: t1.t2_id

Hits: 900006 Misses: 99994 Evictions: 0 Overflows: 0 Memory
Usage: 10547kB

-> Aggregate (cost=4.64..4.65 rows=1 width=8) (actual
time=0.003..0.003 rows=1 loops=99994)

-> Index Only Scan using t2_id_idx on t2 (cost=0.42..4.62
rows=11 width=0) (actual time=0.002..0.003 rows=10 loops=99994)

Index Cond: (id = t1.t2_id)

Heap Fetches: 0

Planning Time: 0.091 ms

Execution Time: 1120.177 ms

when work_mem is 10MB

postgres=# set work_mem to '10MB'; -- 11MB is ok
SET
postgres=# explain analyze select * from t1, lateral(select count(*) from
t2 where t1.t2_id = t2.id) s ;
QUERY PLAN

--------------------------------------------------------------------------------------------------------------------------------------------
Nested Loop (cost=4.65..472639.79 rows=1000000 width=16) (actual
time=0.040..56576.187 rows=1000000 loops=1)
-> Seq Scan on t1 (cost=0.00..14425.00 rows=1000000 width=8) (actual
time=0.010..76.753 rows=1000000 loops=1)
-> Result Cache (cost=4.65..4.67 rows=1 width=8) (actual
time=0.056..0.056 rows=1 loops=1000000)
Cache Key: t1.t2_id
Hits: 884158 Misses: 115842 Evictions: 18752 Overflows: 0
Memory Usage: 10241kB
-> Aggregate (cost=4.64..4.65 rows=1 width=8) (actual
time=0.005..0.005 rows=1 loops=115842)
-> Index Only Scan using t2_id_idx on t2 (cost=0.42..4.62
rows=11 width=0) (actual time=0.003..0.004 rows=10 loops=115842)
Index Cond: (id = t1.t2_id)
Heap Fetches: 0
Planning Time: 0.087 ms
Execution Time: 56621.421 ms
(11 rows)

The query without result cache

postgres=# explain analyze select * from t1, lateral(select count(*) from
t2 where t1.t2_id = t2.id) s ;
QUERY PLAN

---------------------------------------------------------------------------------------------------------------------------------------
Nested Loop (cost=4.64..4689425.00 rows=1000000 width=16) (actual
time=0.031..3260.858 rows=1000000 loops=1)
-> Seq Scan on t1 (cost=0.00..14425.00 rows=1000000 width=8) (actual
time=0.008..71.792 rows=1000000 loops=1)
-> Aggregate (cost=4.64..4.65 rows=1 width=8) (actual
time=0.003..0.003 rows=1 loops=1000000)
-> Index Only Scan using t2_id_idx on t2 (cost=0.42..4.62
rows=11 width=0) (actual time=0.002..0.002 rows=10 loops=1000000)
Index Cond: (id = t1.t2_id)
Heap Fetches: 0
Planning Time: 0.081 ms
Execution Time: 3293.543 ms
(8 rows)

Samples: 119K of event 'cycles', 4000 Hz, Event count (approx.):
Overhead Shared Object Symbol
79.20% postgres [.] cache_reduce_memory
1.94% [kernel] [k] native_write_msr_safe
1.63% [kernel] [k] update_cfs_shares
1.00% [kernel] [k] trigger_load_balance
0.97% [kernel] [k] timerqueue_add
0.51% [kernel] [k] task_tick_fair
0.51% [kernel] [k] task_cputime
0.50% [kernel] [k] perf_event_task_tick
0.50% [kernel] [k] update_curr
0.49% [kernel] [k] hrtimer_active

Regards

Pavel