Re: [HACKERS] An issue in remote query optimization

[Tom Lane]

Abbas Butt  writes:
> Postgres_fdw optimizes remote queries by pushing down the where clause.
> This feature does not work consistently when the query is executed from
> within a pl/pgsql function. The optimization works when the function
> executes the query for the first 5 times, and fails afterwards.

Well, it's switching to a generic plan.  Your first five executions look
like (ignoring the startup transient):

> 2017-01-31 00:39:25 PST LOG:  duration: 0.315 ms  plan:
> Query Text: select bfrom foreign_numbers where a=x
> Foreign Scan on public.foreign_numbers  (cost=100.00..111.91 rows=1
> width=516)
>   Output: b
>   Remote SQL: SELECT b FROM public.numbers WHERE ((a = 2))

with actual durations ranging from 0.250 to 0.315 ms.  After that
you get generic plans:

> 2017-01-31 00:39:25 PST LOG:  duration: 0.251 ms  plan:
> Query Text: select bfrom foreign_numbers where a=x
> Foreign Scan on public.foreign_numbers  (cost=100.00..114.91 rows=1
> width=516)
>   Output: b
>   Filter: (foreign_numbers.a = $3)
>   Remote SQL: SELECT a, b FROM public.numbers

with actual durations ranging from 0.223 to 0.251 ms.

So where's the slowdown, exactly?  It looks to me like the planner
has concluded that a custom plan is not enough better than generic
to justify repeated planning cost, and it looks to me like it was right.

> Note that the remote query does not contain the WHERE clause after the 5th
> invocation.

You'd have to take that up with the author of the FDW you're using.
Optimization of queries involving foreign tables is almost completely
on the head of the FDW, and this one seems not to know about pushing
down WHERE clauses that involve Param nodes.

regards, tom lane


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Re: [HACKERS] An issue in remote query optimization

[Abbas Butt]

Sorry for the confusion.
ANALYZE works for the foreign table 'foreign_numbers'.
test=# analyze foreign_numbers;
ANALYZE
test=#



On Tue, Jan 31, 2017 at 5:04 AM, Ashutosh Bapat <
ashutosh.ba...@enterprisedb.com> wrote:

> On Tue, Jan 31, 2017 at 5:23 PM, Abbas Butt 
> wrote:
> >
> >
> > On Tue, Jan 31, 2017 at 3:15 AM, Etsuro Fujita <
> fujita.ets...@lab.ntt.co.jp>
> > wrote:
> >>
> >> On 2017/01/31 19:53, Abbas Butt wrote:
> >>>
> >>> On Tue, Jan 31, 2017 at 2:25 AM, Etsuro Fujita
> >>> mailto:fujita.ets...@lab.ntt.co.jp>>
> wrote:
> >>> On 2017/01/31 18:24, Abbas Butt wrote:
> >>
> >>
> >>> Postgres_fdw optimizes remote queries by pushing down the where
> >>> clause.
> >>> This feature does not work consistently when the query is
> >>> executed from
> >>> within a pl/pgsql function. The optimization works when the
> >>> function
> >>> executes the query for the first 5 times, and fails afterwards.
> >>
> >>
> >>> I understand that this is because PostgreSQL starts using
> >>> generic plan
> >>> with pulled up where clause after the 5th invocation hoping
> that
> >>> it
> >>> would be faster since we have skiped planning the query on each
> >>> invocation, but in this case this decision is causing the query
> >>> to slow
> >>> down.
> >>
> >>
> >>> How should we fix this problem?
> >>
> >>
> >>> ANALYZE for the foreign table doesn't work?
> >>
> >>
> >>> No.
> >>>
> >>> analyze ts.tickets;
> >>> WARNING:  skipping "tickets" --- cannot analyze this foreign table
> >>> ANALYZE
> >>
> >>
> >> How the foreign table ts.tickets is defined?
> >
> >
> > test=# \d ts.tickets
> >  Foreign table "ts.tickets"
> >  Column |  Type   | Modifiers | FDW Options
> > +-+---+-
> >  id | integer | not null  |
> > Server: mysql_server
> > FDW Options: (dbname 'msql_test_db', table_name 'tickets')
> >
> > Its a foreign table, referring to table 'tickets' defined on MySQL.
> >
> Isn't your original complaint about postgres_fdw? You can not tickets,
> which is a mongo_fdw foreign table, is probably because mongo_fdw has
> not implemented analyze FDW routine.
>
> --
> Best Wishes,
> Ashutosh Bapat
> EnterpriseDB Corporation
> The Postgres Database Company
>



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Re: [HACKERS] An issue in remote query optimization

[Ashutosh Bapat]

On Tue, Jan 31, 2017 at 5:23 PM, Abbas Butt  wrote:
>
>
> On Tue, Jan 31, 2017 at 3:15 AM, Etsuro Fujita 
> wrote:
>>
>> On 2017/01/31 19:53, Abbas Butt wrote:
>>>
>>> On Tue, Jan 31, 2017 at 2:25 AM, Etsuro Fujita
>>> mailto:fujita.ets...@lab.ntt.co.jp>> wrote:
>>> On 2017/01/31 18:24, Abbas Butt wrote:
>>
>>
>>> Postgres_fdw optimizes remote queries by pushing down the where
>>> clause.
>>> This feature does not work consistently when the query is
>>> executed from
>>> within a pl/pgsql function. The optimization works when the
>>> function
>>> executes the query for the first 5 times, and fails afterwards.
>>
>>
>>> I understand that this is because PostgreSQL starts using
>>> generic plan
>>> with pulled up where clause after the 5th invocation hoping that
>>> it
>>> would be faster since we have skiped planning the query on each
>>> invocation, but in this case this decision is causing the query
>>> to slow
>>> down.
>>
>>
>>> How should we fix this problem?
>>
>>
>>> ANALYZE for the foreign table doesn't work?
>>
>>
>>> No.
>>>
>>> analyze ts.tickets;
>>> WARNING:  skipping "tickets" --- cannot analyze this foreign table
>>> ANALYZE
>>
>>
>> How the foreign table ts.tickets is defined?
>
>
> test=# \d ts.tickets
>  Foreign table "ts.tickets"
>  Column |  Type   | Modifiers | FDW Options
> +-+---+-
>  id | integer | not null  |
> Server: mysql_server
> FDW Options: (dbname 'msql_test_db', table_name 'tickets')
>
> Its a foreign table, referring to table 'tickets' defined on MySQL.
>
Isn't your original complaint about postgres_fdw? You can not tickets,
which is a mongo_fdw foreign table, is probably because mongo_fdw has
not implemented analyze FDW routine.

-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company


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Re: [HACKERS] An issue in remote query optimization

[Abbas Butt]

On Tue, Jan 31, 2017 at 3:15 AM, Etsuro Fujita 
wrote:

> On 2017/01/31 19:53, Abbas Butt wrote:
>
>> On Tue, Jan 31, 2017 at 2:25 AM, Etsuro Fujita
>> mailto:fujita.ets...@lab.ntt.co.jp>> wrote:
>> On 2017/01/31 18:24, Abbas Butt wrote:
>>
>
> Postgres_fdw optimizes remote queries by pushing down the where
>> clause.
>> This feature does not work consistently when the query is
>> executed from
>> within a pl/pgsql function. The optimization works when the
>> function
>> executes the query for the first 5 times, and fails afterwards.
>>
>
> I understand that this is because PostgreSQL starts using
>> generic plan
>> with pulled up where clause after the 5th invocation hoping that
>> it
>> would be faster since we have skiped planning the query on each
>> invocation, but in this case this decision is causing the query
>> to slow
>> down.
>>
>
> How should we fix this problem?
>>
>
> ANALYZE for the foreign table doesn't work?
>>
>
> No.
>>
>> analyze ts.tickets;
>> WARNING:  skipping "tickets" --- cannot analyze this foreign table
>> ANALYZE
>>
>
> How the foreign table ts.tickets is defined?
>

test=# \d ts.tickets
 Foreign table "ts.tickets"
 Column |  Type   | Modifiers | FDW Options
+-+---+-
 id | integer | not null  |
Server: mysql_server
FDW Options: (dbname 'msql_test_db', table_name 'tickets')

Its a foreign table, referring to table 'tickets' defined on MySQL.


> Best regards,
> Etsuro Fujita
>
>
>


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Re: [HACKERS] An issue in remote query optimization

[Etsuro Fujita]

On 2017/01/31 19:53, Abbas Butt wrote:

On Tue, Jan 31, 2017 at 2:25 AM, Etsuro Fujita
mailto:fujita.ets...@lab.ntt.co.jp>> wrote:
On 2017/01/31 18:24, Abbas Butt wrote:



Postgres_fdw optimizes remote queries by pushing down the where
clause.
This feature does not work consistently when the query is
executed from
within a pl/pgsql function. The optimization works when the function
executes the query for the first 5 times, and fails afterwards.



I understand that this is because PostgreSQL starts using
generic plan
with pulled up where clause after the 5th invocation hoping that it
would be faster since we have skiped planning the query on each
invocation, but in this case this decision is causing the query
to slow
down.



How should we fix this problem?



ANALYZE for the foreign table doesn't work?



No.

analyze ts.tickets;
WARNING:  skipping "tickets" --- cannot analyze this foreign table
ANALYZE


How the foreign table ts.tickets is defined?

Best regards,
Etsuro Fujita




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Re: [HACKERS] An issue in remote query optimization

[Abbas Butt]

On Tue, Jan 31, 2017 at 2:25 AM, Etsuro Fujita 
wrote:

> On 2017/01/31 18:24, Abbas Butt wrote:
>
>> Postgres_fdw optimizes remote queries by pushing down the where clause.
>> This feature does not work consistently when the query is executed from
>> within a pl/pgsql function. The optimization works when the function
>> executes the query for the first 5 times, and fails afterwards.
>>
>
> I understand that this is because PostgreSQL starts using generic plan
>> with pulled up where clause after the 5th invocation hoping that it
>> would be faster since we have skiped planning the query on each
>> invocation, but in this case this decision is causing the query to slow
>> down.
>>
>> How should we fix this problem?
>>
>
> ANALYZE for the foreign table doesn't work?
>

No.

analyze ts.tickets;
WARNING:  skipping "tickets" --- cannot analyze this foreign table
ANALYZE



>
> Best regards,
> Etsuro Fujita
>
>
>


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Skype ID: gabbasb
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Re: [HACKERS] An issue in remote query optimization

[Etsuro Fujita]

On 2017/01/31 18:24, Abbas Butt wrote:

Postgres_fdw optimizes remote queries by pushing down the where clause.
This feature does not work consistently when the query is executed from
within a pl/pgsql function. The optimization works when the function
executes the query for the first 5 times, and fails afterwards.



I understand that this is because PostgreSQL starts using generic plan
with pulled up where clause after the 5th invocation hoping that it
would be faster since we have skiped planning the query on each
invocation, but in this case this decision is causing the query to slow
down.

How should we fix this problem?


ANALYZE for the foreign table doesn't work?

Best regards,
Etsuro Fujita




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[HACKERS] An issue in remote query optimization

[Abbas Butt]

Hi,
Postgres_fdw optimizes remote queries by pushing down the where clause.
This feature does not work consistently when the query is executed from
within a pl/pgsql function. The optimization works when the function
executes the query for the first 5 times, and fails afterwards.

Example:
Step 1:
Create the table on the foreign server and insert some rows in it
create table numbers(a int, b varchar(255));
insert into numbers values(1, 'One');
insert into numbers values(2, 'Two');
insert into numbers values(3, 'Three');
insert into numbers values(4, 'Four');
insert into numbers values(5, 'Five');
insert into numbers values(6, 'Six');
insert into numbers values(7, 'Seven');
insert into numbers values(8, 'Eight');
insert into numbers values(9, 'Nine');

Step 2:
Create the following objects on the local server
CREATE SERVER pg_server FOREIGN DATA WRAPPER postgres_fdw OPTIONS (host
'127.0.0.1', port '5552', dbname 'test');
CREATE USER MAPPING FOR abbasbutt SERVER pg_server OPTIONS (user
'abbasbutt', password 'abc123');
CREATE FOREIGN TABLE foreign_numbers(a int, b varchar(255)) SERVER
pg_server OPTIONS (table_name 'numbers');

create or replace function test_pg_fdw() returns void as $$
DECLARE
 n varchar;
BEGIN
 FOR x IN 1..9 LOOP
 select b into n from foreign_numbers where a=x;
 raise notice 'Found number %', n;
 end loop;
 return;
END
$$ LANGUAGE plpgsql;

Step 3:
Run the test:
select test_pg_fdw();

Step 4:
Check the output of auto_explain in the local server log

2017-01-31 00:39:25 PST LOG:  duration: 8.388 ms  plan:
Query Text: select bfrom foreign_numbers where a=x
Foreign Scan on public.foreign_numbers  (cost=100.00..111.91 rows=1
width=516)
  Output: b
  Remote SQL: SELECT b FROM public.numbers WHERE ((a = 1))
2017-01-31 00:39:25 PST CONTEXT:  SQL statement "select bfrom
foreign_numbers where a=x"
PL/pgSQL function test_pg_fdw() line 6 at SQL statement
2017-01-31 00:39:25 PST LOG:  duration: 0.315 ms  plan:
Query Text: select bfrom foreign_numbers where a=x
Foreign Scan on public.foreign_numbers  (cost=100.00..111.91 rows=1
width=516)
  Output: b
  Remote SQL: SELECT b FROM public.numbers WHERE ((a = 2))
2017-01-31 00:39:25 PST CONTEXT:  SQL statement "select bfrom
foreign_numbers where a=x"
PL/pgSQL function test_pg_fdw() line 6 at SQL statement
2017-01-31 00:39:25 PST LOG:  duration: 0.250 ms  plan:
Query Text: select bfrom foreign_numbers where a=x
Foreign Scan on public.foreign_numbers  (cost=100.00..111.91 rows=1
width=516)
  Output: b
  Remote SQL: SELECT b FROM public.numbers WHERE ((a = 3))
2017-01-31 00:39:25 PST CONTEXT:  SQL statement "select bfrom
foreign_numbers where a=x"
PL/pgSQL function test_pg_fdw() line 6 at SQL statement
2017-01-31 00:39:25 PST LOG:  duration: 0.257 ms  plan:
Query Text: select bfrom foreign_numbers where a=x
Foreign Scan on public.foreign_numbers  (cost=100.00..111.91 rows=1
width=516)
  Output: b
  Remote SQL: SELECT b FROM public.numbers WHERE ((a = 4))
2017-01-31 00:39:25 PST CONTEXT:  SQL statement "select bfrom
foreign_numbers where a=x"
PL/pgSQL function test_pg_fdw() line 6 at SQL statement
2017-01-31 00:39:25 PST LOG:  duration: 0.271 ms  plan:
Query Text: select bfrom foreign_numbers where a=x
Foreign Scan on public.foreign_numbers  (cost=100.00..111.91 rows=1
width=516)
  Output: b
  Remote SQL: SELECT b FROM public.numbers WHERE ((a = 5))
2017-01-31 00:39:25 PST CONTEXT:  SQL statement "select bfrom
foreign_numbers where a=x"
PL/pgSQL function test_pg_fdw() line 6 at SQL statement
2017-01-31 00:39:25 PST LOG:  duration: 0.251 ms  plan:
Query Text: select bfrom foreign_numbers where a=x
Foreign Scan on public.foreign_numbers  (cost=100.00..114.91 rows=1
width=516)
  Output: b
  Filter: (foreign_numbers.a = $3)
  Remote SQL: SELECT a, b FROM public.numbers
2017-01-31 00:39:25 PST CONTEXT:  SQL statement "select bfrom
foreign_numbers where a=x"
PL/pgSQL function test_pg_fdw() line 6 at SQL statement
2017-01-31 00:39:25 PST LOG:  duration: 0.246 ms  plan:
Query Text: select bfrom foreign_numbers where a=x
Foreign Scan on public.foreign_numbers  (cost=100.00..114.91 rows=1
width=516)
  Output: b
  Filter: (foreign_numbers.a = $3)
  Remote SQL: SELECT a, b FROM public.numbers
2017-01-31 00:39:25 PST CONTEXT:  SQL statement "select bfrom
foreign_numbers where a=x"
PL/pgSQL function test_pg_fdw() line 6 at SQL statement
2017-01-31 00:39:25 PST LOG:  duration: 0.226 ms  plan:
Query Text: select bfrom foreign_numbers where a=x
Foreign Scan on public.foreign_numbers  (cost=100.00..114.91 rows=1
width=516)
  Output: b
  Filter: (foreign_numbers.a = $3)
  Remote SQL: SELECT a, b FROM public.numbers
2017-01-31 00:39:25 PST CO