MongoDB Foreign Data Wrapper for PostgreSQL ============================================ This PostgreSQL extension implements a Foreign Data Wrapper (FDW) for [MongoDB][1]. Please note that this version of mongo_fdw works with PostgreSQL, EDB Postgres Advanced Server, and EDB Postgres Extended Server 14, 15, 16, 17, 18, and 19. Contents -------- 1. [Features](#features) 2. [Supported platforms](#supported-platforms) 3. [Installation](#installation) 4. [Usage](#usage) 5. [Functions](#functions) 6. [Character set handling](#character-set-handling) 7. [Examples](#examples) 8. [Limitations](#limitations) 9. [Contributing](#contributing) 10. [Support](#support) 11. [Useful links](#useful-links) 12. [License](#license) 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][1] connection for every query. This is a performance enhancement. #### WHERE clause push-down WHERE clauses on foreign tables are now pushed down to the remote MongoDB server. By executing WHERE clauses directly on the foreign server, we significantly reduce data transfer to PostgreSQL, resulting in improved query performance. #### 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`. * `mongo_fdw.log_remote_query`: If `true`, logs the remote query that would be executed on MongoDB. Intended for debugging purposes only. Default is `false`. 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](INSTALL.md). If you run into any issues, please [let us know][2]. 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](https://www.postgresql.org/docs/current/sql-importforeignschema.html) 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: - **mongo_fdw_version()** Returns the version number as an integer. 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. 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. ```sql CREATE EXTENSION mongo_fdw; ``` ### Create a foreign server with appropriate configuration: Once for a foreign data source you need, as PostgreSQL superuser. ```sql 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). ```sql 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: ```sql 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: ```sql -- 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][1]'s equivalent statements. #### `SELECT` ```sql 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` ```sql 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` ```sql DELETE FROM warehouse WHERE warehouse_id = 2; ``` ``` db.warehouse.remove ( { "warehouse_id" : 2 } ) ``` #### `UPDATE` ```sql 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` ```sql EXPLAIN SELECT * FROM warehouse WHERE warehouse_id = 1; ``` ``` QUERY PLAN ----------------------------------------------------------------- Foreign Scan on warehouse (cost=0.00..0.00 rows=1000 width=84) 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. - Column names containing a dollar sign (`$`) at the start or immediately following a dot are not supported (e.g., `$field`, `nested.$field`). While MongoDB allows storing these field names, they cannot be referenced in aggregation pipeline expressions, causing queries to return empty results. Column names with `$` in any other position (e.g., `field$name`, `col$`) function normally. - When aggregate functions (e.g., `SUM`, `AVG`) over `SMALLINT`/`INTEGER`/ `BIGINT` columns are pushed down to MongoDB, the `NaN`/`Infinity` validation applied during normal row fetches does not apply to the aggregate result. PostgreSQL computes such aggregates as `NUMERIC`, which can legitimately hold `NaN`/`Infinity`, so if the underlying MongoDB documents contain a `NaN` or `Infinity` value in that field, the aggregate may silently return `NaN`/`Infinity` instead of erroring, even though selecting the column directly correctly raises an error for the same data. Setting `mongo_fdw.enable_aggregate_pushdown` to `false` forces the aggregate to be computed locally by PostgreSQL, which applies the same validation as a normal row fetch. Contributing ------------ Have a fix for a bug or an idea for a great new feature? Great! Check out the contribution guidelines [here][3]. 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`. Useful links ------------ ### Documentation - For details, please refer to [mongo_fdw documentation][5]. ### 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/ License ------- Portions Copyright (c) 2004-2026, 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`][4] file for full details. [1]: http://www.mongodb.com [2]: https://github.com/enterprisedb/mongo_fdw/issues/new [3]: CONTRIBUTING.md [4]: LICENSE [5]: https://www.enterprisedb.com/docs/mongo_data_adapter/latest/