Example: Join pushdown v2

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This example shows join pushdown between the foreign tables of the same remote HIVE/SPARK server to that remote HIVE/SPARK server:

Tables on HIVE/SPARK server:

0: jdbc:hive2://localhost:10000> describe emp;
+-----------+------------+----------+--+
| col_name  | data_type  | comment  |
+-----------+------------+----------+--+
| empno     | int        | NULL     |
| ename     | string     | NULL     |
| job       | string     | NULL     |
| mgr       | int        | NULL     |
| hiredate  | date       | NULL     |
| sal       | int        | NULL     |
| comm      | int        | NULL     |
| deptno    | int        | NULL     |
+-----------+------------+----------+--+
8 rows selected (0.747 seconds)
0: jdbc:hive2://localhost:10000> describe dept;
+-----------+------------+----------+--+
| col_name  | data_type  | comment  |
+-----------+------------+----------+--+
| deptno    | int        | NULL     |
| dname     | string     | NULL     |
| loc       | string     | NULL     |
+-----------+------------+----------+--+
3 rows selected (0.067 seconds)

Tables on Postgres server:

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

-- create server object
CREATE SERVER hdfs_server FOREIGN DATA WRAPPER hdfs_fdw OPTIONS(host 'localhost', port '10000', client_type 'spark', auth_type 'LDAP');

-- create user mapping
CREATE USER MAPPING FOR public SERVER hdfs_server OPTIONS (username 'user1', password 'pwd123');

-- create foreign table
CREATE FOREIGN TABLE dept (
    deptno          INTEGER,
    dname           VARCHAR(14),
    loc             VARCHAR(13)
)
SERVER hdfs_server OPTIONS (dbname 'fdw_db', table_name 'dept');

-- create foreign table
CREATE FOREIGN TABLE emp (
    empno           INTEGER,
    ename           VARCHAR(10),
    job             VARCHAR(9),
    mgr             INTEGER,
    hiredate        DATE,
    sal             INTEGER,
    comm            INTEGER,
    deptno          INTEGER
)
SERVER hdfs_server OPTIONS (dbname 'fdw_db', table_name 'emp');

Queries with join pushdown:

--inner join
edb=# EXPLAIN VERBOSE SELECT t1.ename, t2.dname FROM emp t1 INNER JOIN dept t2 ON ( t1.deptno = t2.deptno );

Output

                                                                QUERY PLAN                                                               
---------------------------------------------------------------------------------------------------------------------------------------
 Foreign Scan  (cost=15.00..35.00 rows=5000 width=84)
   Output: t1.ename, t2.dname
   Relations: (fdw_db.emp t1) INNER JOIN (fdw_db.dept t2)
   Remote SQL: SELECT r1., r2. FROM (. r1 INNER JOIN . r2 ON (((r1. = r2.))))
 (4 rows)

--left join
edb=# EXPLAIN VERBOSE SELECT t1.ename, t2.dname FROM emp t1 LEFT JOIN dept t2 ON ( t1.deptno = t2.deptno );

Output

                                                              QUERY PLAN                                                              
--------------------------------------------------------------------------------------------------------------------------------------
 Foreign Scan  (cost=15.00..35.00 rows=5000 width=84)
   Output: t1.ename, t2.dname
   Relations: (fdw_db.emp t1) LEFT JOIN (fdw_db.dept t2)
   Remote SQL: SELECT r1., r2. FROM (. r1 LEFT JOIN . r2 ON (((r1. = r2.))))
(4 rows)

--right join
edb=# EXPLAIN VERBOSE SELECT t1.ename, t2.dname FROM emp t1 RIGHT JOIN dept t2 ON ( t1.deptno = t2.deptno );

Output

                                                              QUERY PLAN                                                              
--------------------------------------------------------------------------------------------------------------------------------------
 Foreign Scan  (cost=15.00..35.00 rows=5000 width=84)
   Output: t1.ename, t2.dname
   Relations: (fdw_db.dept t2) LEFT JOIN (fdw_db.emp t1)
   Remote SQL: SELECT r1., r2. FROM (. r2 LEFT JOIN . r1 ON (((r1. = r2.))))
(4 rows)

--full join
edb=# EXPLAIN VERBOSE SELECT t1.ename, t2.dname FROM emp t1 FULL JOIN dept t2 ON ( t1.deptno = t2.deptno );

Output

                                                              QUERY PLAN                                                              
--------------------------------------------------------------------------------------------------------------------------------------
 Foreign Scan  (cost=15.00..35.00 rows=5000 width=84)
   Output: t1.ename, t2.dname
   Relations: (fdw_db.emp t1) FULL JOIN (fdw_db.dept t2)
   Remote SQL: SELECT r1., r2. FROM (. r1 FULL JOIN . r2 ON (((r1. = r2.))))
(4 rows)

--cross join
edb=# EXPLAIN VERBOSE SELECT t1.ename, t2.dname FROM emp t1 CROSS JOIN dept t2;

Output

                                                  QUERY PLAN                                                  
--------------------------------------------------------------------------------------------------------------
 Foreign Scan  (cost=15.00..35.00 rows=1000000 width=84)
   Output: t1.ename, t2.dname
   Relations: (fdw_db.emp t1) INNER JOIN (fdw_db.dept t2)
   Remote SQL: SELECT r1., r2. FROM (. r1 INNER JOIN . r2 ON (TRUE))
(4 rows)

Enable/disable GUC for join pushdown queries at the table level:

-- enable join pushdown at the table level
ALTER FOREIGN TABLE emp OPTIONS (SET enable_join_pushdown 'true');
EXPLAIN (VERBOSE, COSTS OFF)
SELECT e.empno, e.ename, d.dname
  FROM emp e JOIN dept d ON (e.deptno = d.deptno)
  ORDER BY e.empno;

Output

                                                                       QUERY PLAN                                                                        
---------------------------------------------------------------------------------------------------------------------------------------------------------
 Sort
   Output: e.empno, e.ename, d.dname
   Sort Key: e.empno
   ->  Foreign Scan
         Output: e.empno, e.ename, d.dname
         Relations: (fdw_db.emp e) INNER JOIN (fdw_db.dept d)
         Remote SQL: SELECT r1., r1., r2. FROM (. r1 INNER JOIN . r2 ON (((r1. = r2.))))
(7 rows)

--Disable the GUC enable_join_pushdown.
SET hdfs_fdw.enable_join_pushdown to false;
-- Pushdown shouldn't happen as enable_join_pushdown is false.
EXPLAIN (VERBOSE, COSTS OFF)
SELECT e.empno, e.ename, d.dname
  FROM emp e JOIN dept d ON (e.deptno = d.deptno)
  ORDER BY e.empno;

Output

                                        QUERY PLAN                                         
-------------------------------------------------------------------------------------------
 Sort
   Output: e.empno, e.ename, d.dname
   Sort Key: e.empno
   ->  Nested Loop
         Output: e.empno, e.ename, d.dname
         Join Filter: (e.deptno = d.deptno)
         ->  Foreign Scan on public.emp e
               Output: e.empno, e.ename, e.job, e.mgr, e.hiredate, e.sal, e.comm, e.deptno
               Remote SQL: SELECT , ,  FROM .
         ->  Materialize
               Output: d.dname, d.deptno
               ->  Foreign Scan on public.dept d
                     Output: d.dname, d.deptno
                     Remote SQL: SELECT ,  FROM .

Enable/disable GUC for join pushdown queries at the session level:

SET hdfs_fdw.enable_join_pushdown to true;
EXPLAIN (VERBOSE, COSTS OFF)
SELECT e.empno, e.ename, d.dname
  FROM emp e JOIN dept d ON (e.deptno = d.deptno)
  ORDER BY e.empno;

Output

                                        QUERY PLAN                                         
-------------------------------------------------------------------------------------------
 Sort
   Output: e.empno, e.ename, d.dname
   Sort Key: e.empno
   ->  Nested Loop
         Output: e.empno, e.ename, d.dname
         Join Filter: (e.deptno = d.deptno)
         ->  Foreign Scan on public.emp e
               Output: e.empno, e.ename, e.job, e.mgr, e.hiredate, e.sal, e.comm, e.deptno
               Remote SQL: SELECT , ,  FROM .
         ->  Materialize
               Output: d.dname, d.deptno
               ->  Foreign Scan on public.dept d
                     Output: d.dname, d.deptno
                     Remote SQL: SELECT ,  FROM .
(14 rows)

Note

You can also enable/disable join pushdown at the server level using a GUC. For more information, see CREATE SERVER.

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