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 10, 11, 12, 13, 14, and 15.

Installation

To compile the MongoDB foreign data wrapper, mongo-c and json-c libraries are needed. To build and install mongo-c and json-c libraries, there are two ways. You can either use script autogen.sh or you can manually perform all required steps listed.

Installation using script

Number of manual steps needs to be performed to compile and install required mongo-c and json-c libraries. If you want to avoid the manual steps, there is a shell script available which will download and install the appropriate drivers and libraries for you.

Here is how it works:

To install mongo-c and json-c libraries at custom locations, you need to export environment variables MONGOC_INSTALL_DIR and JSONC_INSTALL_DIR respectively. If these variables are not set then these libraries will be installed in the default location. Please note that you need to have the required permissions on the directory where you want to install the libraries.

Build with MongoDB's legacy branch driver (Deprecated in mongo_fdw 5.4.0)

• autogen.sh --with-legacy

Build MongoDB's master branch driver

• autogen.sh --with-master

The script autogen.sh will do all the necessary steps to build with legacy and meta driver accordingly.

Steps for manual installation

mongo-c

meta driver

1. Download and extract source code of mongoc driver for version 1.17.3

   wget https://github.com/mongodb/mongo-c-driver/releases/download/1.17.3/mongo-c-driver-1.17.3.tar.gz
   tar xzf mongo-c-driver-1.17.3.tar.gz
   rm -rf mongo-c-driver
   mv mongo-c-driver-1.17.3 mongo-c-driver
   cd mongo-c-driver

2. Configure mongoc driver

   cmake -DENABLE_AUTOMATIC_INIT_AND_CLEANUP=OFF .

   To install at custom location:

   cmake -DCMAKE_INSTALL_PREFIX=YOUR_INSTALLATION_DIRECTORY -DENABLE_AUTOMATIC_INIT_AND_CLEANUP=OFF .

3. Compile and install

   cmake --build .
   cmake --build . --target install

For more details on installation of mongo-c driver, you can refer here.

Legacy driver

Deprecation Notice: The legacy driver support has been deprecated in mongo_fdw 5.4.0 and is expected to be removed entirely in a future release.

• Checkout, extract legacy branch

   wget https://github.com/mongodb/mongo-c-driver/archive/v0.8.tar.gz
   tar -zxf v0.8.tar.gz
   rm -rf mongo-c-driver
   mv  mongo-c-driver-0.8 mongo-c-driver

json-c

1. Download and extract source code

   wget https://github.com/json-c/json-c/archive/json-c-0.15-20200726.tar.gz
   tar -xzf json-c-0.15-20200726.tar.gz
   rm -rf json-c
   mv json-c-json-c-0.15-20200726/ json-c
   cd json-c

2. Configure

   cmake .

   To install at custom location:

   cmake -DCMAKE_INSTALL_PREFIX=YOUR_INSTALLATION_DIRECTORY .

3. Compile and install

   make
   make install

For more details on installation of json-c library, you can refer here.

How to compile against mongo-c Meta or Legacy driver?

To compile against legacy driver, 'Makefile.legacy' must be used and 'Makefile.meta' must be used to compile against the meta driver. For example, this can be achieved by copying required Makefile as shown below: For meta,

cp Makefile.meta Makefile

For legacy (Deprecated in mongo_fdw 5.4.0),

cp Makefile.legacy Makefile

The default compilation is with Meta driver.

Mongo_fdw configuration, compilation and installation

The PKG_CONFIG_PATH environment variable must be set to mongo-c-driver source directory for successful compilation as shown below,

export PKG_CONFIG_PATH=$YOUR_MONGO_FDW_SOURCE_DIR/mongo-c-driver/src/libmongoc/src:$YOUR_MONGO_FDW_SOURCE_DIR/mongo-c-driver/src/libbson/src

The LD_LIBRARY_PATH environment variable must include the path to the mongo-c installation directory containing the libmongoc-1.0.so and libbson-1.0.so files. For example, assuming the installation directory is /home/mongo-c and the libraries were created under it in lib64 sub-directory, then we can define the LD_LIBRARY_PATH as:

export LD_LIBRARY_PATH=/home/mongo-c/lib64:$LD_LIBRARY_PATH

Note: This LD_LIBRARY_PATH environment variable setting must be in effect when the pg_ctl utility is executed to start or restart PostgreSQL or EDB Postgres Advanced Server.

1. To build on POSIX-compliant systems you need to ensure the pg_config executable is in your path when you run make. This executable is typically in your PostgreSQL installation's bin directory. For example:

   export PATH=/usr/local/pgsql/bin/:$PATH

2. Compile the code using make.

   make USE_PGXS=1

3. Finally install the foreign data wrapper.

   make USE_PGXS=1 install

4. Running regression test.

   make USE_PGXS=1 installcheck

   However, make sure to set the MONGO_HOST, MONGO_PORT, MONGO_USER_NAME, and MONGO_PWD environment variables correctly. The default settings can be found in the mongodb_init.sh script.

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

Enhancements

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 MySQL 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.

New MongoDB C Driver Support

This enhancement is to add a new MongoDB' C driver. The current implementation is based on the legacy driver of MongoDB. But MongoDB is provided completely new library for driver called MongoDB's meta driver. Added support for the same. Now compile time option is available to use legacy and meta driver.

In order to use MongoDB driver 1.17.0+, take the following steps:

• clone libmongoc version 1.17.0+ (https://github.com/mongodb/mongo-c-driver) and follow the install directions given there. libbson is now maintained in a subdirectory of the libmongoc. (https://github.com/mongodb/mongo-c-driver/tree/master/src/libbson).
• ensure pkg-config / pkgconf is installed on your system.
• run make -f Makefile.meta && make -f Makefile.meta install
• if you get an error when trying to CREATE EXTENSION mongo_fdw;, then try running ldconfig

Usage

The following parameters can be set on a MongoDB foreign server object:

• address: Address or hostname of the MongoDB server. Defaults to 127.0.0.1
• port: Port number of the MongoDB server. Defaults to 27017.
• use_remote_estimate: Controls whether mongo_fdw uses exact rows from remote collection to obtain cost estimates. Default is false.

The following options are only supported with meta driver:

• authentication_database: Database against which user will be authenticated against. Only valid with password based authentication.
• replica_set: Replica set the server is member of. If set, driver will auto-connect to correct primary in the replica set when writing.
• read_preference: primary [default], secondary, primaryPreferred, secondaryPreferred, or nearest.
• ssl: false [default], true to enable ssl. See http://mongoc.org/libmongoc/current/mongoc_ssl_opt_t.html to understand the options.
• pem_file: The .pem file that contains both the TLS/SSL certificate and key.
• pem_pwd: The password to decrypt the certificate key file(i.e. pem_file)
• ca_file: The .pem file that contains the root certificate chain from the Certificate Authority.
• ca_dir: The absolute path to the ca_file.
• crl_file: The .pem file that contains the Certificate Revocation List.
• weak_cert_validation: false [default], This is to enable or disable the validation checks for TLS/SSL certificates and allows the use of invalid certificates to connect if set to true.
• 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. 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. Default is true.
• enable_aggregate_pushdown: 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. Default is true.

The following parameters can be set on a MongoDB foreign table object:

• database: Name of the MongoDB database to query. Defaults to test.
• collection: Name of the MongoDB collection to query. Defaults to the foreign table name used in the relevant CREATE command.
• enable_join_pushdown: Similar to the server-level option, but can be configured at table level as well. Default is true.
• enable_aggregate_pushdown: Similar to the server-level option, but can be configured at table level as well. Default is true.

The following parameters can be supplied while creating user mapping:

• username: Username to use when connecting to MongoDB.
• password: Password to authenticate to the MongoDB server.

GUC variables:

• 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.

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.

Examples

Examples with MongoDB's equivalent statements.

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

-- create server object
CREATE SERVER mongo_server
	FOREIGN DATA WRAPPER mongo_fdw
	OPTIONS (address '127.0.0.1', port '27017');

-- create user mapping
CREATE USER MAPPING FOR postgres
	SERVER mongo_server
	OPTIONS (username 'mongo_user', password 'mongo_pass');

-- create foreign table
CREATE FOREIGN TABLE warehouse
	(
		_id name,
		warehouse_id int,
		warehouse_name text,
		warehouse_created timestamptz
	)
	SERVER mongo_server
	OPTIONS (database 'db', collection 'warehouse');

-- Note: first column of the table must be "_id" of type "name".

-- select from table
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 row in table
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 row from table
DELETE FROM warehouse WHERE warehouse_id = 2;

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

-- update a row of table
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 a table
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)

-- collect data distribution statistics
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.

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) 2004-2022, 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.