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https://github.com/EnterpriseDB/mongo_fdw

PostgreSQL foreign data wrapper for MongoDB
https://github.com/EnterpriseDB/mongo_fdw

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PostgreSQL foreign data wrapper for MongoDB

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README 

        
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 and EDB

Postgres Advanced Server 12, 13, 14, 15, 16 and 17.



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.



#### 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](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)

   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][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) 2004-2024, 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`][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/