postgres_fdw: possible to push data to foreign server for join?

问题

suppose I have a query like

select * from remote_table
   join local_table using(common_key)

where remote_table is a FOREIGN TABLE with postgres_fdw and local_table is a regular table.

local_table is small (100 rows) and remote_table is large (millions of rows).

It looks like the remote table is pulled in its entirety and joined locally, when it would be more efficient to ship the smaller table to the remote server and join remotely.

Is there a way to get postgres_fdw to do that?

回答1:

You cannot do that with a join, since joins between tables on different servers are always executed locally.

What you could try is something like:

SELECT *
FROM (SELECT *
      FROM remote_table
      WHERE common_key IN (SELECT common_key FROM local_table)
     ) a
  JOIN local_table USING (common_key);

I did not test it, so I am not sure if it will work, but the idea is to use a condition for the foreign table scan that can be pushed down and reduces the amount of data fetched as much as possible.

回答2:

Have you tried deploying the local data into a temp table on the foreign server then joining it into the foreign table? Not sure of your process or if this would be efficient for you or not.

来源：https://stackoverflow.com/questions/51970227/postgres-fdw-possible-to-push-data-to-foreign-server-for-join