A PostgreSQL Interface for XML Data using the CData ODBC Driver

There are a vast number of PostgreSQL clients available on the Internet. From standard Drivers to BI and Analytics tools, PostgreSQL is a popular interface for data access. Using the SQL Gateway included in our ODBC Drivers, you can now create PostgreSQL entry-points that you can connect to from any standard client.

To access XML data as a PostgreSQL database on Windows, use the CData SQL Gateway, the ODBC Driver for XML, and the MySQL foreign data wrapper from EnterpriseDB. In this article, we compile the foreign data wrapper in Visual Studio, install it as an extension, and query XML data from PostgreSQL Server.

Configure the Connection to XML

If you have not already, first specify connection properties in an ODBC DSN (data source name). This is the last step of the driver installation. You can use the Microsoft ODBC Data Source Administrator to create and configure ODBC DSNs.

Connecting to Local or Cloud-Stored (Box, Google Drive, Amazon S3, SharePoint) XML Files

CData Drivers let you work with XML files stored locally and stored in cloud storage services like Box, Amazon S3, Google Drive, or SharePoint, right where they are.

Setting connection properties for local files

Set the URI property to local folder path.

Setting connection properties for files stored in Amazon S3

To connect to XML file(s) within Amazon S3, set the URI property to the URI of the Bucket and Folder where the intended XML files exist. In addition, at least set these properties:

• AWSAccessKey: AWS Access Key (username)
• AWSSecretKey: AWS Secret Key

Setting connection properties for files stored in Box

To connect to XML file(s) within Box, set the URI property to the URI of the folder that includes the intended XML file(s). Use the OAuth authentication method to connect to Box.

Dropbox

To connect to XML file(s) within Dropbox, set the URI proprerty to the URI of the folder that includes the intended XML file(s). Use the OAuth authentication method to connect to Dropbox. Either User Account or Service Account can be used to authenticate.

SharePoint Online (SOAP)

To connect to XML file(s) within SharePoint with SOAP Schema, set the URI proprerty to the URI of the document library that includes the intended XML file. Set User, Password, and StorageBaseURL.

SharePoint Online REST

To connect to XML file(s) within SharePoint with REST Schema, set the URI proprerty to the URI of the document library that includes the intended XML file. StorageBaseURL is optional. If not set, the driver will use the root drive. OAuth is used to authenticate.

Google Drive

To connect to XML file(s) within Google Drive, set the URI property to the URI of the folder that includes the intended XML file(s). Use the OAuth authentication method to connect and set InitiateOAuth to GETANDREFRESH.

The property is the controlling property over how your data is represented into tables and toggles the following basic configurations.

• Document (default): Model a top-level, document view of your XML data. The data provider returns nested elements as aggregates of data.
• FlattenedDocuments: Implicitly join nested documents and their parents into a single table.
• Relational: Return individual, related tables from hierarchical data. The tables contain a primary key and a foreign key that links to the parent document.

See the Modeling XML Data chapter for more information on configuring the relational representation. You will also find the sample data used in the following examples. The data includes entries for people, the cars they own, and various maintenance services performed on those cars.

Start the Remoting Service

The MySQL remoting service is a daemon process that listens for clients' incoming MySQL connections. See the setup guide in the SQL Gateway overview to configure the MySQL Remoting service in the CData SQL Gateway.

Build the MySQL Foreign Data Wrapper

The Foreign Data Wrapper can be installed as an extension to PostgreSQL, without recompiling PostgreSQL. If you are running PostgreSQL on a Unix-based system, you can use the PostgreSQL Extension Network (PGXN) to install the FDW, mysql_fdw. If you are running PostgreSQL on Windows, compile the extension to ensure that you are working with the latest version. Follow the steps below to make the necessary modifications to build the extension from Visual Studio:

Obtain Prerequisites

To build the foreign data wrapper, do the following:

• Install PostgreSQL. This example uses an installation of PostgreSQL 9.4.
• Obtain the source for the mysql_fdw foreign data wrapper from EnterpriseDB.
• Install MySQL Connector C. This example uses an installation of MySQL Connector C 6.1.

Configure a Project

After you have obtained the necessary software and source code, you are ready to compile the extension with Visual Studio. Follow the steps below to create a project using the mysql_fdw source:

1. In Visual Studio, create a new empty C++ project.
2. In Solution Explorer, right-click Source Files and click Add -> Existing Item. In the file explorer, select all of the .c and .h files from the mysql_fdw source.

Follow the steps below to configure your project:

1. If you are building for a 64-bit system, click Build -> Configuration Manager and in Active Solution Platform select x64.
2. Right-click your project and click Properties.
3. In the Configuration menu, select All Configurations.
4. In Configuration Properties -> General -> Configuration Type, select Dynamic Library.
5. In Configuration Properties -> C/C++ -> Code Generation -> Enable C++ Exceptions, select No.
6. In Configuration Properties -> C/C++ -> Advanced -> Compile As, select Compile as C Code.
7. In Linker -> Manifest File -> Generate Manifest, select No.

Follow the steps below to add the required dependencies:

1. In Linker -> Input -> Additional Dependencies, select Edit and enter the following:

   postgres.lib
   libmysql.lib
   WS2_32.lib
   Secur32.lib
   

   Additionally, ensure that Inherit From Parent or Project Defaults is checked.
2. In Linker -> General -> Additional Library Directories, select Edit and add
   • the path to the lib folder in your PostgreSQL installation
   • the path to the lib folder for your MySQL Connector C installation
3. In Linker -> General -> Link Library Dependencies, select No.
4. To complete the configuration of your project, add the necessary includes: In C/C++ -> General -> Additional Include Directories, add the following folders in the following order:

   MyMySQLConnectorCInstallation\include
   MyPostgreSQLInstallation\MyPostgreSQLVersion\include\server\port\win32_msvc
   MyPostgreSQLInstallation\MyPostgreSQLVersion\include\server\port\win32
   MyPostgreSQLInstallation\MyPostgreSQLVersion\include\server
   MyPostgreSQLInstallation\MyPostgreSQLVersion\include
   

Configure mysql_fdw for Windows

After setting up a project, make the following changes to build mysql_fdw in Visual Studio:

1. In mysql_fdw.c, add the following defines:

   #define dlsym(lib, name) (void*)GetProcAddress((HMODULE)lib, name)
   #define dlopen(libname, unused) LoadLibraryEx(libname, NULL, 0)
   

2. In the mysql_load_library definition, delete the following line:

   mysql_dll_handle = dlopen(_MYSQL_LIBNAME, RTLD_LAZY | RTLD_DEEPBIND);
   

3. Add the following line in the mysql_load_library definition to replace the assignment of mysql_dll_handle for a Windows build:

   mysql_dll_handle = dlopen("libmysql.dll", 0);
   

4. Prepend the call to the mysql_fdw_handler function with the __declspec(dllexport) keyword to export the function from the DLL:

   __declspec(dllexport) extern Datum mysql_fdw_handler(PG_FUNCTION_ARGS);

5. In option.c, prepend the declaration of the mysql_fdw_validator function with the __declspec(dllexport) keyword to export the function from the DLL:

   __declspec(dllexport) extern Datum mysql_fdw_validator(PG_FUNCTION_ARGS);

You can now select the Release configuration and build.

Install the Extension

After you have compiled the DLL, follow the steps below to install the extension:

1. Add the path to the lib folder for MySQL Connector C to the PATH environment variable of the machine running PostgreSQL.
2. Copy the DLL from the RElease folder for your project into the lib subfolder of your PostgreSQL installation.
3. In the folder containing the mysql_fdw csource files, copy myswl_fdw--1.0.sql and mysql_fdw.control into the extension folder under the share folder of your PostgreSQL installation. For example: C:\Program Files\PostgreSQL\9.4\share\extension.

Query XML Data as a PostgreSQL Database

After you have installed the extension, follow the steps below to start executing queries to XML data:

1. Log into your PostgreSQL database. For example:

   C:\> psql -U postgres
   

2. Load the extension for the database:

   postgres=#CREATE EXTENSION mysql_fdw;
   

3. Create a server object for XML data:

   postgres=# CREATE SERVER XML FOREIGN DATA WRAPPER mysql_fdw OPTIONS (host
   '127.0.0.1', port '3306');
   

4. Create a user mapping for the username and password of a user known to the MySQL remoting service. Below are the credentials for the user in the sample configuration of the service:

   postgres=# CREATE USER MAPPING for postgres SERVER XML OPTIONS (username
   'admin', password 'test');
   

5. Create the local schema:

   postgres=# CREATE SCHEMA XML_db;

6. Import all the tables in the XML database you defined:

   postgres=# IMPORT FOREIGN SCHEMA "CData XML Sys" FROM SERVER XML INTO XML_db;
   

You can now execute read/write commands to XML:

postgres=# SELECT * FROM XML_db."people";