MongoDB Foreign Data Wrapper for PostgreSQL

This PostgreSQL extension implements a Foreign Data Wrapper (FDW) for MongoDB.

Please note that this version of mongo_fdw works with PostgreSQL and EDB Postgres Advanced Server 12, 13, 14, 15 and 16.

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Contents

1. Features
2. Supported platforms
3. Installation
4. Usage
5. Functions
6. Identifier case handling
7. Generated columns
8. Character set handling
9. Examples
10. Limitations
11. Contributing
12. Useful links

Features

The following enhancements are added to the latest version of mongo_fdw:

Write-able FDW

The previous version was only read-only, the latest version provides the write capability. The user can now issue an insert, update, and delete statements for the foreign tables using the mongo_fdw.

Connection Pooling

The latest version comes with a connection pooler that utilizes the same MongoDB database connection for all the queries in the same session. The previous version would open a new MongoDB connection for every query. This is a performance enhancement.

JOIN push-down

mongo_fdw now also supports join push-down. The joins between two foreign tables from the same remote MongoDB server are pushed to a remote server, instead of fetching all the rows for both the tables and performing a join locally, thereby may enhance the performance. Currently, joins involving only relational and arithmetic operators in join-clauses are pushed down to avoid any potential join failure. Also, only the INNER and LEFT/RIGHT OUTER joins are supported, and not the FULL OUTER, SEMI, and ANTI join. Moreover, only joins between two tables are pushed down and not when either inner or outer relation is the join itself.

AGGREGATE push-down

mongo_fdw now also supports aggregate push-down. Push aggregates to the remote MongoDB server instead of fetching all of the rows and aggregating them locally. This gives a very good performance boost for the cases where aggregates can be pushed down. The push-down is currently limited to aggregate functions min, max, sum, avg, and count, to avoid pushing down the functions that are not present on the MongoDB server. The aggregate filters, orders, variadic and distinct are not pushed down.

ORDER BY push-down

mongo_fdw now also supports order by push-down. If possible, push order by clause to the remote server so that we get the ordered result set from the foreign server itself. It might help us to have an efficient merge join. NULLs behavior is opposite on the MongoDB server. Thus to get an equivalent result, we can only push-down ORDER BY with either ASC NULLS FIRST or DESC NULLS LAST. Moreover, as MongoDB sorts only on fields, only column names in ORDER BY expressions are pushed down.

LIMIT OFFSET push-down

mongo_fdw now also supports limit offset push-down. Wherever possible, perform LIMIT and OFFSET operations on the remote server. This reduces network traffic between local PostgreSQL and remote MongoDB servers.

GUC variables:

• mongo_fdw.enable_join_pushdown: If true, pushes the join between two foreign tables from the same foreign server, instead of fetching all the rows for both the tables and performing a join locally. Default is true.
• mongo_fdw.enable_aggregate_pushdown: If true, pushes aggregate operations to the foreign server, instead of fetching rows from the foreign server and performing the operations locally. Default is true.
• mongo_fdw.enable_order_by_pushdown: If true, pushes the order by operation to the foreign server, instead of fetching rows from the foreign server and performing the sort locally. Default is true.

Supported platforms

mongo_fdw was developed on Linux, and should run on any reasonably POSIX-compliant system.

Installation

About script or manual installation, mongo-c driver please read the following instructions in INSTALL.md.

If you run into any issues, please let us know.

Usage

CREATE SERVER options

mongo_fdw accepts the following options via the CREATE SERVER command:

• address as string, optional, default 127.0.0.1

  Address or hostname of the MongoDB server.

• port as integer, optional, default 27017.

  Port number of the MongoDB server.

• use_remote_estimate as boolean, optional, default false

  Controls whether mongo_fdw uses exact rows from remote collection to obtain cost estimates.

• authentication_database as string, optional

  Database against which user will be authenticated against. Only valid with password based authentication.

• replica_set as string, optional

  Replica set the server is member of. If set, driver will auto-connect to correct primary in the replica set when writing.

• read_preference as string, optional, default primary

  primary, secondary, primaryPreferred, secondaryPreferred, or nearest.

• ssl as boolean, optional, default false

  Enable ssl. See http://mongoc.org/libmongoc/current/mongoc_ssl_opt_t.html to understand the options.

• pem_file as string, optional

  The .pem file that contains both the TLS/SSL certificate and key.

• pem_pwd as string, optional

  The password to decrypt the certificate key file(i.e. pem_file)

• ca_file as string, optional

  The .pem file that contains the root certificate chain from the Certificate Authority.

• ca_dir as string, optional

  The absolute path to the ca_file.

• crl_file as string, optional

  The .pem file that contains the Certificate Revocation List.

• weak_cert_validation as boolean, optional, default false

  Enable the validation checks for TLS/SSL certificates and allows the use of invalid certificates to connect if set to true.

• enable_join_pushdown as boolean, optional, default true

  If true, pushes the join between two foreign tables from the same foreign server, instead of fetching all the rows for both the tables and performing a join locally. This option can also be set for an individual table, and if any of the tables involved in the join has set it to false then the join will not be pushed down. The table-level value of the option takes precedence over the server-level option value.

• enable_aggregate_pushdown as boolean, optional, default true

  If true, push aggregates to the remote MongoDB server instead of fetching all of the rows and aggregating them locally. This option can also be set for an individual table. The table-level value of the option takes precedence over the server-level option value.

• enable_order_by_pushdown as boolean, optional, default true

  If true, pushes the ORDER BY clause to the foreign server instead of performing a sort locally. This option can also be set for an individual table, and if any of the tables involved in the query has set it to false then the ORDER BY will not be pushed down. The table-level value of the option takes precedence over the server-level option value.

CREATE USER MAPPING options

mongo_fdw accepts the following options via the CREATE USER MAPPING command:

• username as string, optional

  Username to use when connecting to MongoDB.

• password as string, optional

  Password to authenticate to the MongoDB server.

CREATE FOREIGN TABLE options

mongo_fdw accepts the following table-level options via the CREATE FOREIGN TABLE command:

• database as string, optional, default test

  Name of the MongoDB database to query.

• collection as string, optional, default name of foreign table

  Name of the MongoDB collection to query.

• enable_join_pushdown as boolean, optional, default true

  Similar to the server-level option, but can be configured at table level as well.

• enable_aggregate_pushdown as boolean, optional, default true

  Similar to the server-level option, but can be configured at table level as well.

• enable_order_by_pushdown as boolean, optional, default true

  Similar to the server-level option, but can be configured at table level as well.

No column-level options are available.

IMPORT FOREIGN SCHEMA options

mongo_fdw don't supports IMPORT FOREIGN SCHEMA because MongoDB is schemaless.

TRUNCATE support

mongo_fdw don't implements the foreign data wrapper TRUNCATE API, available from PostgreSQL 14, because MongoDB is schemaless.

Functions

As well as the standard mongo_fdw_handler() and mongo_fdw_validator() functions, mongo_fdw provides the following user-callable utility functions:

Yet not described!.

Identifier case handling

PostgreSQL folds identifiers to lower case by default, MongoDB use JSON notation of identifiers.

All transformation rules and problems yet not described.

Generated columns

mongo_fdw doesn't provides support for PostgreSQL's generated columns (PostgreSQL 12+).

Behaviour with generated columns yet not tested and not described.

Note that while mongo_fdw will INSERT or UPDATE the generated column value in MongoDB, there is nothing to stop the value being modified within MongoDB, and hence no guarantee that in subsequent SELECT operations the column will still contain the expected generated value. This limitation also applies to postgres_fdw.

For more details on generated columns see:

• Generated Columns
• CREATE FOREIGN TABLE

Character set handling

BSON in MongoDB can only be encoded in UTF-8. Also UTF-8 is recommended and de-facto most popular PostgreSQL server encoding.

Encodings mapping between PostgreSQL and MongoDB yet not described.

Examples

As an example, the following commands demonstrate loading the mongo_fdw wrapper, creating a server, and then creating a foreign table associated with a MongoDB collection. The commands also show specifying option values in the OPTIONS clause. If an option value isn't provided, the wrapper uses the default value mentioned above.

mongo_fdw can collect data distribution statistics will incorporate them when estimating costs for the query execution plan. To see selected execution plans for a query, just run EXPLAIN.

Install the extension:

Once for a database you need, as PostgreSQL superuser.

CREATE EXTENSION mongo_fdw;

Create a foreign server with appropriate configuration:

Once for a foreign data source you need, as PostgreSQL superuser.

CREATE SERVER "MongoDB server" FOREIGN DATA WRAPPER mongo_fdw OPTIONS (
  address '127.0.0.1',
  port '27017'
);

Grant usage on foreign server to normal user in PostgreSQL:

Once for a normal user (non-superuser) in PostgreSQL, as PostgreSQL superuser. It is a good idea to use a superuser only where really necessary, so let's allow a normal user to use the foreign server (this is not required for the example to work, but it's security recommendation).

GRANT USAGE ON FOREIGN SERVER "MongoDB server" TO pguser;

Where pguser is a sample user for works with foreign server (and foreign tables).

User mapping

Create an appropriate user mapping:

CREATE USER MAPPING FOR pguser SERVER "MongoDB server" OPTIONS (
  username 'mongo_user',
  password 'mongo_pass'
);

Where pguser is a sample user for works with foreign server (and foreign tables).

Create foreign table

All CREATE FOREIGN TABLE SQL commands can be executed as a normal PostgreSQL user if there were correct GRANT USAGE ON FOREIGN SERVER. No need of PostgreSQL supersuer for security reasons but also works with PostgreSQL supersuer.

Create a foreign table referencing the MongoDB collection:

-- Note: first column of the table must be "_id" of type "name".
CREATE FOREIGN TABLE warehouse (
  _id name,
  warehouse_id int,
  warehouse_name text,
  warehouse_created timestamptz
) SERVER "MongoDB server" OPTIONS (
    database 'db',
	collection 'warehouse'
);

Typical examples with MongoDB's equivalent statements.

SELECT

SELECT * FROM warehouse WHERE warehouse_id = 1;

           _id            | warehouse_id | warehouse_name |     warehouse_created
--------------------------+--------------+----------------+---------------------------
 53720b1904864dc1f5a571a0 |            1 | UPS            | 2014-12-12 12:42:10+05:30
(1 row)

db.warehouse.find
(
	{
		"warehouse_id" : 1
	}
).pretty()
{
	"_id" : ObjectId("53720b1904864dc1f5a571a0"),
	"warehouse_id" : 1,
	"warehouse_name" : "UPS",
	"warehouse_created" : ISODate("2014-12-12T07:12:10Z")
}

INSERT

INSERT INTO warehouse VALUES (0, 2, 'Laptop', '2015-11-11T08:13:10Z');
-- Note: The given value for "_id" column will be ignored and allows MongoDB to
-- insert the unique value for the "_id" column.

db.warehouse.insert
(
	{
		"warehouse_id" : NumberInt(2),
		"warehouse_name" : "Laptop",
		"warehouse_created" : ISODate("2015-11-11T08:13:10Z")
	}
)

DELETE

DELETE FROM warehouse WHERE warehouse_id = 2;

db.warehouse.remove
(
	{
		"warehouse_id" : 2
	}
)

UPDATE

UPDATE warehouse SET warehouse_name = 'UPS_NEW' WHERE warehouse_id = 1;

db.warehouse.update
(
	{
		"warehouse_id" : 1
	},
	{
		"warehouse_id" : 1,
		"warehouse_name" : "UPS_NEW",
		"warehouse_created" : ISODate("2014-12-12T07:12:10Z")
	}
)

EXPLAIN, ANALYZE

EXPLAIN SELECT * FROM warehouse WHERE warehouse_id = 1;

                           QUERY PLAN
-----------------------------------------------------------------
 Foreign Scan on warehouse  (cost=0.00..0.00 rows=1000 width=84)
   Filter: (warehouse_id = 1)
   Foreign Namespace: db.warehouse
(3 rows)

ANALYZE warehouse;

Limitations

• If the BSON document key contains uppercase letters or occurs within a nested document, mongo_fdw requires the corresponding column names to be declared in double quotes.

• Note that PostgreSQL limits column names to 63 characters by default. If you need column names that are longer, you can increase the NAMEDATALEN constant in src/include/pg_config_manual.h, compile, and re-install.

Contributing

Have a fix for a bug or an idea for a great new feature? Great! Check out the contribution guidelines here.

Useful links

Documentation

• For details, please refer to mongo_fdw documentation.

Source code

Reference FDW realization, postgres_fdw

• https://git.postgresql.org/gitweb/?p=postgresql.git;a=tree;f=contrib/postgres_fdw;hb=HEAD

General FDW Documentation

• https://www.postgresql.org/docs/current/ddl-foreign-data.html
• https://www.postgresql.org/docs/current/sql-createforeigndatawrapper.html
• https://www.postgresql.org/docs/current/sql-createforeigntable.html
• https://www.postgresql.org/docs/current/sql-importforeignschema.html
• https://www.postgresql.org/docs/current/fdwhandler.html
• https://www.postgresql.org/docs/current/postgres-fdw.html

Other FDWs

• https://wiki.postgresql.org/wiki/Fdw
• https://pgxn.org/tag/fdw/

Support

This project will be modified to maintain compatibility with new PostgreSQL and EDB Postgres Advanced Server releases.

If you need commercial support, please contact the EnterpriseDB sales team, or check whether your existing PostgreSQL support provider can also support mongo_fdw.

License

Portions Copyright (c) 2004-2024, EnterpriseDB Corporation. Portions Copyright © 2012–2014 Citus Data, Inc.

This program is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

See the LICENSE file for full details.