Re: [ADMIN] ORDER BY too slow in Foreign Table using postgres_fdw

Hi Luan,

 

Sort operation is performed on local postgres server, not on remote. Because of that, local postgresql server gets all rows matched by filter and then sort them. If your code always sort the results, you can just create a view on remote postgres with order by clause and then create a foreign table pointing to the view. By using a view like that, sorting is performed on remote server.

 

On remote Postgres

create view user_info_vw as select id,info from user_info order by id;

 

On local Postgres

create foreign table user_info (id bigint, info jsonb) server luan_server options (schema_name 'public', table_name 'user_info_vw');

 

 

Before view

postgres=# explain analyze SELECT id, info

FROM user_info

WHERE info ->> 'key1'= '1' order by id limit 10;

                                                           QUERY PLAN                                                            

---------------------------------------------------------------------------------------------------------------------------------

 Limit  (cost=153.28..153.29 rows=6 width=40) (actual time=7512.755..7512.759 rows=10 loops=1)

   ->  Sort  (cost=153.28..153.29 rows=6 width=40) (actual time=7512.754..7512.757 rows=10 loops=1)

         Sort Key: id

         Sort Method: top-N heapsort  Memory: 26kB

         ->  Foreign Scan on user_info  (cost=100.00..153.20 rows=6 width=40) (actual time=0.962..7351.989 rows=1187840 loops=1)

               Filter: ((info ->> 'key1'::text) = '1'::text)

               Rows Removed by Filter: 786432

 Planning time: 0.089 ms

 Execution time: 7513.322 ms

(9 rows)

 

 

After view

explain analyze SELECT id, info

FROM user_info

WHERE info ->> 'key1'= '1' LIMIT 10;

                                                    QUERY PLAN                                                     

-------------------------------------------------------------------------------------------------------------------

 Limit  (cost=100.00..153.20 rows=6 width=40) (actual time=0.678..0.684 rows=10 loops=1)

   ->  Foreign Scan on user_info  (cost=100.00..153.20 rows=6 width=40) (actual time=0.677..0.681 rows=10 loops=1)

         Filter: ((info ->> 'key1'::text) = '1'::text)

         Rows Removed by Filter: 4

 Planning time: 0.060 ms

 Execution time: 1.167 ms

(6 rows)

 

postgres=# SELECT id, info

postgres-# FROM user_info

postgres-# WHERE info ->> 'key1'= '1' LIMIT 10;

 id |            info            

----+----------------------------

  1 | {"key1": 1, "key2": 0.678}

  2 | {"key1": 1, "key2": 0.678}

  3 | {"key1": 1, "key2": 1.0}

  4 | {"key1": 1, "key2": 0.986}

  7 | {"key1": 1, "key2": 0.678}

  8 | {"key1": 1, "key2": 1.0}

  9 | {"key1": 1, "key2": 0.986}

 12 | {"key1": 1, "key2": 0.678}

 13 | {"key1": 1, "key2": 1.0}

 14 | {"key1": 1, "key2": 0.986}

(10 rows)

 

Best regards.

İyi çalışmalar.

Samed YILDIRIM

18.07.2017, 10:06, "Luan Huynh" <nnhluan@gmail.com>:

Hi all, 

On PostgreSQL v.9.6, when using postgres_fdw, I got an issue with "ORDER BY" (here's my question on stackexchange ).

Query on Foreign Table

SELECT id, info 

FROM user_info

WHERE info ->> 'key1'= '1' 

ORDER BY id 

LIMIT 10; 

Limit  (cost=10750829.63..10750829.65 rows=10 width=40) (actual time=550059.320..550059.326 rows=10 loops=1)

   ->  Sort  (cost=10750829.63..10751772.77 rows=377257 width=40) (actual time=550059.318..550059.321 rows=10 loops=1)

         Sort Key: id

         Sort Method: top-N heapsort  Memory: 26kB

         ->  Foreign Scan on user_info (cost=100.00..10742677.24 rows=377257 width=40) (actual time=1.413..536718.366 rows=68281020 loops=1)

               Filter: ((info ->> 'key1'::text) = '1'::text)

               Rows Removed by Filter: 7170443

 Planning time: 4.097 ms

 Execution time: 550059.597 ms

Query on remote server

EXPLAIN ANALYSE

SELECT id, info 

FROM user_info_raw

WHERE info ->> 'key1'= '1'

ORDER BY id 

LIMIT 10;

 Limit  (cost=0.57..1296.95 rows=10 width=59) (actual time=0.043..0.073 rows=10 loops=1)

   ->  Index Scan using idx_user_info_raw_info on user_info_raw  (cost=0.57..68882850.88 rows=531346 width=59) (actual time=0.042..0.070 rows=10 loops=1)

         Filter: ((info ->> 'key1'::text) = '1'::text)

 Planning time: 0.192 ms

 Execution time: 0.102 ms 

Please help me to figure out the solution for that issue .

Thank you