How to connect PolyBase to HDFS

PolyBase for SQL Server allows you to query external data by using the same Transact-SQL syntax used to query a database table. When paired with the CData ODBC Driver for HDFS, you get access to your HDFS data directly alongside your SQL Server data. This article describes creating an external data source and external tables to grant access to live HDFS data using T-SQL queries.

NOTE: PolyBase is only available on SQL Server 19 and above, and only for Standard SQL Server.

The CData ODBC drivers offer unmatched performance for interacting with live HDFS data using PolyBase due to optimized data processing built into the driver. When you issue complex SQL queries from SQL Server to HDFS, the driver pushes down supported SQL operations, like filters and aggregations, directly to HDFS and utilizes the embedded SQL engine to process unsupported operations (often SQL functions and JOIN operations) client-side. And with PolyBase, you can also join SQL Server data with HDFS data, using a single query to pull data from distributed sources.

Connect to HDFS

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. To create an external data source in SQL Server using PolyBase, configure a System DSN (CData HDFS Sys is created automatically).

In order to authenticate, set the following connection properties:

• Host: Set this value to the host of your HDFS installation.
• Port: Set this value to the port of your HDFS installation. Default port: 50070

Click "Test Connection" to ensure that the DSN is connected to HDFS properly. Navigate to the Tables tab to review the table definitions for HDFS.

Create an External Data Source for HDFS Data

After configuring the connection, you need to create a master encryption key and a credential database for the external data source.

Creating a Master Encryption Key

Execute the following SQL command to create a new master key, 'ENCRYPTION,' to encrypt the credentials for the external data source.

CREATE MASTER KEY ENCRYPTION BY PASSWORD = 'password';

Creating a Credential Database

Execute the following SQL command to create credentials for the external data source connected to HDFS data.

CREATE DATABASE SCOPED CREDENTIAL hdfs_creds
WITH IDENTITY = 'username', SECRET = 'password';

Create an External Data Source for HDFS

Execute a CREATE EXTERNAL DATA SOURCE SQL command to create an external data source for HDFS with PolyBase:

• Set the LOCATION parameter , using the DSN and credentials configured earlier.

For HDFS, set SERVERNAME to the URL or address for your server (e.g. 'localhost' or '127.0.0.1' for local servers; the remote URL for remote servers). Leave PORT empty. PUSHDOWN is set to ON by default, meaning the ODBC Driver can leverage server-side processing for complex queries.

CREATE EXTERNAL DATA SOURCE cdata_hdfs_source
WITH ( 
  LOCATION = 'odbc://SERVER_URL',
  CONNECTION_OPTIONS = 'DSN=CData HDFS Sys',
  -- PUSHDOWN = ON | OFF,
  CREDENTIAL = hdfs_creds
);

Create External Tables for HDFS

After creating the external data source, use CREATE EXTERNAL TABLE statements to link to HDFS data from your SQL Server instance. The table column definitions must match those exposed by the CData ODBC Driver for HDFS. You can refer to the Tables tab of the DSN Configuration Wizard to see the table definition.

Sample CREATE TABLE Statement

The statement to create an external table based on a HDFS Files would look similar to the following:

CREATE EXTERNAL TABLE Files(
  FileId [nvarchar](255) NULL,
  ChildrenNum [nvarchar](255) NULL,
  ...
) WITH ( 
  LOCATION='Files',
  DATA_SOURCE=cdata_hdfs_source
);

Having created external tables for HDFS in your SQL Server instance, you are now able to query local and remote data simultaneously. Thanks to built-in query processing in the CData ODBC Driver, you know that as much query processing as possible is being pushed to HDFS, freeing up local resources and computing power. Download a free, 30-day trial of the ODBC Driver for HDFS and start working with live HDFS data alongside your SQL Server data today.