Example: Aggregate function pushdown v5

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MongoDB Foreign Data Wrapper supports pushdown for the following aggregate functions:

  • AVG - Calculates the average of a set of values.
  • COUNT - Counts rows in a specified table or view.
  • MIN - Gets the minimum value in a set of values.
  • MAX - Gets the maximum value in a set of values.
  • SUM - Calculates the sum of values.

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, deptid INTEGER) SERVER mongo_server OPTIONS (database 'edb', collection 'emp');

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

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

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

Query with aggregate pushdown:

-- COUNT function
edb# EXPLAIN VERBOSE SELECT COUNT(*) FROM emp;
Output
                    QUERY PLAN                    
--------------------------------------------------
 Foreign Scan  (cost=15.00..25.00 rows=1 width=8)
   Output: (count(*))
   Foreign Namespace: Aggregate on (db1.emp)
(3 rows)
-- SUM function
edb# EXPLAIN VERBOSE SELECT SUM(deptid) FROM emp;
Output
                    QUERY PLAN                    
--------------------------------------------------
 Foreign Scan  (cost=15.00..25.00 rows=1 width=8)
   Output: (sum(deptid))
   Foreign Namespace: Aggregate on (db1.emp)
(3 rows)
-- AVG function
edb# EXPLAIN VERBOSE SELECT AVG(deptid) FROM emp;
Output
                    QUERY PLAN                     
---------------------------------------------------
 Foreign Scan  (cost=15.00..25.00 rows=1 width=32)
   Output: (avg(deptid))
   Foreign Namespace: Aggregate on (db1.emp)
(3 rows)
-- MAX function
edb# EXPLAIN VERBOSE SELECT MAX(eid) FROM emp;
Output
                    QUERY PLAN                    
--------------------------------------------------
 Foreign Scan  (cost=15.00..25.00 rows=1 width=4)
   Output: (max(eid))
   Foreign Namespace: Aggregate on (db1.emp)
(3 rows)
-- MIN function
edb# EXPLAIN VERBOSE SELECT MIN(eid) FROM emp;
Output
                    QUERY PLAN                    
--------------------------------------------------
 Foreign Scan  (cost=15.00..25.00 rows=1 width=4)
   Output: (min(eid))
   Foreign Namespace: Aggregate on (db1.emp)
(3 rows)
-- MIN and SUM functions with GROUPBY
edb# EXPLAIN VERBOSE SELECT MIN(deptid), SUM(eid) FROM emp GROUP BY deptid HAVING MAX(eid) > 120;
Output
                     QUERY PLAN                      
-----------------------------------------------------
 Foreign Scan  (cost=15.00..25.00 rows=200 width=16)
   Output: (min(deptid)), (sum(eid)), deptid
   Foreign Namespace: Aggregate on (db1.emp)
(3 rows)
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