Example: Join pushdown v5

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MongoDB Foreign Data Wrapper supports pushdown for inner joins, left joins, and right joins. For example:

Postgres data set:

-- 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);

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

-- insert into table
INSERT INTO dept VALUES (0, 10, 'SALES');
INSERT INTO dept VALUES (0, 20, 'ENGG');
INSERT INTO dept VALUES (0, 30, 'IT');

Enable/disable GUC for JOIN pushdown queries at the session level, table level, or server level:

-- Session level
edb=# SET mongo_fdw.enable_join_pushdown to true;
SET
-- Table level
edb=# ALTER FOREIGN TABLE emp OPTIONS (ADD enable_join_pushdown 'true');
Table altered
-- Server level
edb=# ALTER SERVER mongo_server OPTIONS (ADD enable_join_pushdown 'true');
altered

Query with JOIN pushdown:

--inner join
edb=# EXPLAIN VERBOSE SELECT e.ename, d.dname FROM emp e INNER JOIN dept d ON (e.deptid = d.deptid);
Output
                        QUERY PLAN                        
----------------------------------------------------------
 Foreign Scan  (cost=15.00..35.00 rows=5000 width=64)
   Output: e.ename, d.dname
   Foreign Namespace: (edb.emp e) INNER JOIN (edb.dept d)
(3 rows)
--left join
edb=# EXPLAIN VERBOSE SELECT e.ename, d.dname FROM emp e LEFT JOIN dept d ON (e.deptid = d.deptid);
Output
                       QUERY PLAN                        
---------------------------------------------------------
 Foreign Scan  (cost=15.00..35.00 rows=5000 width=64)
   Output: e.ename, d.dname
   Foreign Namespace: (edb.emp e) LEFT JOIN (edb.dept d)
(3 rows)
--right join
edb=# EXPLAIN VERBOSE SELECT e.ename, d.dname FROM emp e RIGHT JOIN dept d ON (e.deptid = d.deptid);
Output
                       QUERY PLAN                        
---------------------------------------------------------
 Foreign Scan  (cost=15.00..35.00 rows=5000 width=64)
   Output: e.ename, d.dname
   Foreign Namespace: (edb.dept d) LEFT JOIN (edb.emp e)
(3 rows)
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