Re: [PERFORM] repeated subplan execution

[monika yadav]

Hi Jeff,

Thanks for the update and clarification. I will look to see a better
alternative to resolve this twice execution of same plan.

On Wed, Sep 20, 2017 at 10:16 PM, Jeff Janes  wrote:

> On Tue, Sep 19, 2017 at 7:31 PM, monika yadav 
> wrote:
>
>> 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 ?
>>
>
>
> The sum_bid at the end of the query is an alias for the entire subselect,
> so it not entirely surprising that it gets interpolated twice. it is just
> kind of unfortunate from a performance perspective.
>
> The query I originally gave is equivalent to this query:
>
>
>  select
> aid,
> (select sum(bid) from pgbench_branches
> where bbalance between -1-abalance and 1+abalance
> ) as sum_bid
> from pgbench_accounts
> where aid between 1 and 1000
> group by aid
> having (select sum(bid) from pgbench_branches where bbalance
> between -1-abalance and 1+abalance ) >0;
>
>
> In my originally query I just wrapped the whole thing in another select,
> so that I could use the alias rather than having to mechanically repeat the
> entire subquery again in the HAVING section.  They give identical plans.
>
> Cheers,
>
> Jeff
>

Re: [PERFORM] repeated subplan execution

[Jeff Janes]

On Tue, Sep 19, 2017 at 7:31 PM, monika yadav 
wrote:

> 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 ?
>


The sum_bid at the end of the query is an alias for the entire subselect,
so it not entirely surprising that it gets interpolated twice. it is just
kind of unfortunate from a performance perspective.

The query I originally gave is equivalent to this query:


 select
aid,
(select sum(bid) from pgbench_branches
where bbalance between -1-abalance and 1+abalance
) as sum_bid
from pgbench_accounts
where aid between 1 and 1000
group by aid
having (select sum(bid) from pgbench_branches where bbalance
between -1-abalance and 1+abalance ) >0;


In my originally query I just wrapped the whole thing in another select, so
that I could use the alias rather than having to mechanically repeat the
entire subquery again in the HAVING section.  They give identical plans.

Cheers,

Jeff

Re: [PERFORM] repeated subplan execution

[monika yadav]

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  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 -1-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 >= ('-1'::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 >= ('-1'::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
>

[PERFORM] repeated subplan execution

[Jeff Janes]

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 -1-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 >= ('-1'::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 >= ('-1'::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