Accessing File-Based External Tables

Accessing File-Based External Tables

External tables enable accessing external files as if they are regular database tables. They are often used to move data into and out of a Greenplum database.

To create an external table definition, you specify the format of your input files and the location of your external data sources. For information input file formats, see Formatting Data Files.

Use one of the following protocols to access external table data sources. You cannot mix protocols in CREATE EXTERNAL TABLE statements:
  • file:// accesses external data files on segment host that the Greenplum superuser (gpadmin) can access. See file:// Protocol.
  • gpfdist:// points to a directory on the file host and serves external data files to all Greenplum Database segments in parallel. See gpfdist:// Protocol.
  • gpfdists:// is the secure version of gpfdist. See gpfdists:// Protocol.
  • gphdfs:// accesses files on a Hadoop Distributed File System (HDFS). See gphdfs:// Protocol.

External tables allow you to access external files from within the database as if they are regular database tables. Used with gpfdist, the Greenplum parallel file distribution program, external tables provide full parallelism by using the resources of all Greenplum segments to load or unload data. Greenplum Database leverages the parallel architecture of the Hadoop Distributed File System to access files on that system.

You can query external table data directly and in parallel using SQL commands such as SELECT, JOIN, or SORT EXTERNAL TABLE DATA, and you can create views for external tables.

The steps for using external tables are:

  1. Define the external table.
  2. Do one of the following:
    • Start the Greenplum file server(s) if you plan to use the gpfdist or gpdists protocols.
    • Verify that you have already set up the required one-time configuration for gphdfs.
  3. Place the data files in the correct locations.
  4. Query the external table with SQL commands.

Greenplum Database provides readable and writable external tables:

  • Readable external tables for data loading. Readable external tables support basic extraction, transformation, and loading (ETL) tasks common in data warehousing. Greenplum Database segment instances read external table data in parallel to optimize large load operations. You cannot modify readable external tables.
  • Writable external tables for data unloading. Writable external tables support:
    • Selecting data from database tables to insert into the writable external table.
    • Sending data to an application as a stream of data. For example, unload data from Greenplum Database and send it to an application that connects to another database or ETL tool to load the data elsewhere.
    • Receiving output from Greenplum parallel MapReduce calculations.

    Writable external tables allow only INSERT operations.

External tables can be file-based or web-based. External tables using the file:// protocol are read-only tables.

  • Regular (file-based) external tables access static flat files. Regular external tables are rescannable: the data is static while the query runs.
  • Web (web-based) external tables access dynamic data sources, either on a web server with the http:// protocol or by executing OS commands or scripts. Web external tables are not rescannable: the data can change while the query runs.

Dump and restore operate only on external and web external table definitions, not on the data sources.