Re: repeated subplan execution

[monika yadav <monika.1234yadav@xxxxxxxxx>]

Hi All,

I didn't understand why same sub plan for the sub query executed two times? As per the query it should have been executed only once.

Can someone please explain this behaviour of query execution ? 

Thanks a lot.

On Wed, 20 Sep 2017 at 5:01 AM, Jeff Janes <jeff.janes@xxxxxxxxx> wrote:

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