DynamoDB Foreign Data Wrapper for PostgreSQL

This is a foreign data wrapper (FDW) to connect PostgreSQL to DynamoDB.

+ DynamoDB

Contents

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

Features

Common features

• Support SELECT feature to get data from DynamoDB.
• DynamoDB FDW supports selecting columns or nested attribute object (using -> or ->> operator)
• Support INSERT feature.
• Support UPDATE feature using foreign modify.
• Support DELETE feature using foreign modify.

Pushdowning

Common pushdowning

• Support push down WHERE clause (including nested attribute object).
• Support push down function SIZE of DynamoDB.
• Does not push down WHERE condition when it compares array constant. For example: SELECT * FROM array_test WHERE array_n < '{1232, 5121, 8438, 644, 83}'; is not pushed down.
• Does not push down WHERE condition when it contains text comparison using <, <=, >=, > operators. For example: SELECT * FROM WHERE name > '@#!S'; is not pushed down.
• Does not push down when selecting multiple attributes with the same name. For example: SELECT name, friends->'class_info'->'name' FROM students;
• Does not push down overlap document path. For example: SELECT friends->'class_info', friends->'class_info'->'name' FROM students;

Comparison operators

No	PostgreSQL	Remark
1	=	Equal to
2	<> or !=	Not Equal to
3	>	Greater than
4	<	Less than
5	>=	Greater than or equal to
6	<=	Less than or equal to

Logical operators

No	PostgreSQL	Remark
1	AND	TRUE if all the conditions separated by AND are TRUE
2	BETWEEN	TRUE if the operand is within the range of comparisons
3	IN	Only support IN with a list of value. Return TRUE if the operand is equal to one of a list of expressions.
4	IS	Only support IS NULL. Return TRUE if the operand is NULL.
5	NOT	Reverses the value of a given Boolean expression
6	OR	TRUE if any of the conditions separated by OR are TRUE

Dereference operators

No	PostgreSQL	Remark
1	->	Extracts JSON object field with the given key. This mapping will be used when right operand is an attribute name (which is represented as a text constant).
2	->>	Extracts JSON object field with the given key, as text.
3	->	Extract nested element of List type. This mapping will be used when right operand is a number.

Notes about features

• For string set and number set of DynamoDB, the values in the set are sorted from smallest to largest automatically. Therefore, if you want to access to the element of array, it will return the different value compared to insert value. For example, you insert <<3,2,1>> into DynamoDB. DynamoDB will sort and store it as <<1,2,3>>. If you want to select element [1] of the set, it will return 1.
• DynamoDB does not support NULL as element of string set or number set. Therefore, when user input NULL as element of array, the default value (0 for number, empty string for text) will be inserted into DynamoDB. For example, user input array[1,2,null,4], the values inserted into DynamoDB will be [1, 2, 0, 4]. User input array['one','two',null,'four'], the values inserted into DynamoDB will be ['one', 'two', '', 'four'].
• If an attribute of Map type does not exist, the condition xxx IS NULL will always return false.

Also see Limitations.

Supported platforms

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

dynamodb_fdw is designed to be compatible with PostgreSQL 13 ~ 16.0.

Installation

Prerequisites

1. AWS C++ SDK DynamoDB FDW uses the APIs provided by AWS C++ SDK to connect and execute query on DynamoDB. It requires gcc version 4.9.0 and above to be able to use and compile. It also requires 3rd party libraries: libcurl, openssl, libuuid, pulseaudio-libs.
2. Java Runtime Environment (JRE) version 8.x or newer If using DynamoDB local, JRE 8.x or newer is required.

Source installation

This section describle how to install required library on CentOS 7.

1. AWS C++ SDK Download and follow the Amazon developer guide.

   https://docs.aws.amazon.com/sdk-for-cpp/v1/developer-guide/setup-linux.html

2. Java Runtime Environment (JRE) version 8.x or newer

    $ sudo yum install java-1.8.0-openjdk-devel.x86_64

3. Build and install PostgreSQL: from PostgreSQL directory

    ./configure
    $ make
    $ make install

4. Build and install DynamoDB FDW: clone source code under PostgreSQL/contrib/

    $ make
    $ make install

Usage

CREATE SERVER options

dynamodb_fdw accepts the following options via the CREATE SERVER command:

• endpoint as string, optional, default http://localhost:8000

  The URL of the entry point for an AWS web service. It is required for AWS DynamoDB and optional for DynamoDB local.

CREATE USER MAPPING options

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

• user as string, optional, no default

  The user credential to connect to DynamoDB. It is required for AWS DynamoDB and optional for DynamoDB local.

• password as string, optional, no default

  The password credential to connect to DynamoDB. It is required for AWS DynamoDB and optional for DynamoDB local.

CREATE FOREIGN TABLE options

dynamodb_fdw accepts the following table-level options via the CREATE FOREIGN TABLE command.

• partition_key as string, optional, no default

  The column name of the partition key of DynamoDB table.

• sort_key as string, optional, no default

  The column name of the sort key of DynamoDB table.

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

  The corresponding table name in DynamoDB.

The following column-level options are available:

• column_name as string, optional, default column name of foreign table

  The corresponding column name in DynamoDB.

IMPORT FOREIGN SCHEMA options

dynamodb_fdw don't support IMPORT FOREIGN SCHEMA and accepts no custom options.

TRUNCATE support

dynamodb_fdw don't support the foreign data wrapper TRUNCATE API, available from PostgreSQL 14.

Functions

As well as the standard dynamodb_fdw_handler() and dynamodb_fdw_validator() functions, dynamodb_fdw provides the following user-callable utility functions: Functions from this FDW in PostgreSQL catalog are yet not described.

Data type mapping

No	PostgreSQL	DynamoDB	Remark
1	boolean	Boolean	N/A
2	bytea	Binary	PartiQL of DynamoDB does not have any way to represent binary data. Therefore, DynamoDB FDW only supports selecting Binary column. DynamoDB FDW does not support Binary column in WHERE clause.
3	JSON/JSONB	Map	N/A
4	NULL	Null	N/A
5	smallint, integer, bigint, numeric, real, double precision	Number	N/A
6	smallint[], integer[], bigint[], numeric[], real[], double precision[]	Number Set	N/A
7	text character varying(n)[], varchar(n)[], character(n)[], char(n) [], text[]	String Set	N/A
8	text character varying(n), varchar(n), character(n), char(n), text	String	N/A

Identifier case handling

PostgreSQL folds identifiers to lower case by default. Rules and problems with DynamoDB identifiers yet not tested and described.

Generated columns

Behaviour within generated columns yet not tested and described.

For more details on generated columns see:

• Generated Columns
• CREATE FOREIGN TABLE

Character set handling

Yet not described.

Examples

Install the extension:

Once for a database you need, as PostgreSQL superuser.

	CREATE EXTENSION dynamodb_fdw;

Create a foreign server with appropriate configuration:

Once for a foreign datasource you need, as PostgreSQL superuser.

	CREATE SERVER dynamodb_svr
	FOREIGN DATA WRAPPER dynamodb_fdw
	OPTIONS (
	  endpoint 'http://localhost:8000'
	);

Grant usage on foreign server to non-superuser in PostgreSQL:

Once for a 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 secirity recomedation).

	GRANT USAGE ON FOREIGN SERVER dynamodb_svr TO pguser;

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

Create an appropriate user mapping:

	CREATE USER MAPPING
	FOR pgser
	SERVER dynamodb_svr 
    	OPTIONS(
	  username 'username',
	  password 'password'
	);

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

Create a foreign table referencing the dynamodb table:

	CREATE FOREIGN TABLE frtbl (
	  c1 int,
	  c2 text,
	  c3 jsonb
	)
	SERVER dynamodb_svr
	OPTIONS (
	  table_name 'table1'
	);

Query the foreign table.

	SELECT *
	FROM frtbl;

Limitations

• Does not support List type of DynamoDB.
• Only support SELECT the Binary type of DynamoDB. Does not support WHERE clause, INSERT, UPDATE statement with Binary type of DynamoDB.
• For DynamoDB, 2 records can have the same attribute name but different data type. However, DynamoDB FDW does not support that case. User need to avoid using that case.
• Does not support COPY FROM and foreign partition routing. The following error will be shown. COPY and foreign partition routing not supported in dynamodb_fdw

Contributing

Opening issues and pull requests on GitHub are welcome.

Useful links

Source code

Reference FDW realisation, 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

Copyright and license information can be found in the file LICENSE.