Example: WHERE clause pushdown v5

-- load extension first time after install
CREATE EXTENSION mongo_fdw;

-- create server object
CREATE SERVER mongo_server FOREIGN DATA WRAPPER mongo_fdw OPTIONS (address 'localhost', port '27017');

-- create user mapping
CREATE USER MAPPING FOR public SERVER mongo_server OPTIONS (username 'edb', password 'edb');

-- create foreign table
CREATE FOREIGN TABLE emp (_id NAME, eid INTEGER, ename TEXT, deptid INTEGER) SERVER mongo_server OPTIONS (database 'edb', collection 'emp');

-- insert into table
INSERT INTO emp VALUES (0, 100, 'John', 10);
INSERT INTO emp VALUES (0, 110, 'Mark', 10);
INSERT INTO emp VALUES (0, 120, 'Smith', 20);
INSERT INTO emp VALUES (0, 130, 'Ed', 30);

edb=# EXPLAIN (VERBOSE, COSTS FALSE) select eid from emp where deptid>20 order by eid;
             QUERY PLAN             
------------------------------------
 Sort
   Output: eid
   Sort Key: emp.eid
   ->  Foreign Scan on public.emp
         Output: eid
         Foreign Namespace: edb.emp
(6 rows)

edb=# 
edb=# select eid from emp where deptid>20 order by eid;
 eid 
-----
 130
(1 row)