[HACKERS] An issue in remote query optimization

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От	Abbas Butt
Тема	[HACKERS] An issue in remote query optimization
Дата	31 января 2017 г. 15:24:31
Msg-id	CALtH27eUC5pZbrcMCDRv9jFdQwxQH+FQPywX=MwP7Tj6Rb-zaw@mail.gmail.com обсуждение исходный текст
Ответы	Re: [HACKERS] An issue in remote query optimization (Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>) Re: [HACKERS] An issue in remote query optimization (Tom Lane <tgl@sss.pgh.pa.us>)
Список	pgsql-hackers

Дерево обсуждения

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 b        from 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 b        from 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 b        from 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 b        from 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 b        from 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 b        from 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 b        from 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 b        from 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 b        from 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 b        from 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 b        from 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 b        from 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 b        from 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 b        from 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 b        from 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 b        from 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: 0.223 ms plan:
    Query Text: select b        from 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 b        from 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: 13.963 ms plan:
    Query Text: select test_pg_fdw();
    Result (cost=0.00..0.26 rows=1 width=0)
    Output: test_pg_fdw()

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

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?

--
Abbas

Architect

Ph: 92.334.5100153

Skype ID: gabbasb

www.enterprisedb.co m

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