I have a complicated query which runs the exact same subplan more than once.
Here is a greatly simplified (and rather pointless) query to replicate the
issue:
select aid, sum_bid from
(select
aid,
(select sum(bid) from pgbench_branches
where bbalance between -10000-abalance and 1+abalance
) as sum_bid
from pgbench_accounts
where aid between 1 and 1000
group by aid
) asdfsadf
where sum_bid >0;
QUERY
PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------
Group (cost=0.44..375841.29 rows=931 width=12) (actual
time=1.233..691.200 rows=679 loops=1)
Group Key: pgbench_accounts.aid
Filter: ((SubPlan 2) > 0)
Rows Removed by Filter: 321
-> Index Scan using pgbench_accounts_pkey on pgbench_accounts
(cost=0.44..634.32 rows=931 width=8) (actual time=0.040..1.783 rows=1000
loops=1)
Index Cond: ((aid >= 1) AND (aid <= 1000))
SubPlan 2
-> Aggregate (cost=403.00..403.01 rows=1 width=8) (actual
time=0.406..0.407 rows=1 loops=1000)
-> Seq Scan on pgbench_branches pgbench_branches_1
(cost=0.00..403.00 rows=1 width=4) (actual time=0.392..0.402 rows=1
loops=1000)
Filter: ((bbalance >= ('-10000'::integer -
pgbench_accounts.abalance)) AND (bbalance <= (1 +
pgbench_accounts.abalance)))
Rows Removed by Filter: 199
SubPlan 1
-> Aggregate (cost=403.00..403.01 rows=1 width=8) (actual
time=0.407..0.407 rows=1 loops=679)
-> Seq Scan on pgbench_branches (cost=0.00..403.00 rows=1
width=4) (actual time=0.388..0.402 rows=1 loops=679)
Filter: ((bbalance >= ('-10000'::integer -
pgbench_accounts.abalance)) AND (bbalance <= (1 +
pgbench_accounts.abalance)))
Rows Removed by Filter: 199
Planning time: 0.534 ms
Execution time: 691.784 ms
https://explain.depesz.com/s/Xaib
The subplan is not so fast that I wish it to be executed again or every row
which passes the filter.
I can prevent this dual execution using a CTE, but that creates other
problems. Is there a way to get rid of it without resorting to that?
Maybe also a question for bugs and/or hackers, is why should I need to do
anything special to avoid dual execution?
Cheers,
Jeff
