Unloading Data from Greenplum Database - Defining a Command-Based Writable External Web Table - 《Greenplum Database v6.24 Documentation》

You can define writable external web tables to send output rows to an application or script. The application must accept an input stream, reside in the same location on all of the Greenplum segment hosts, and be executable by the gpadmin user. All segments in the Greenplum system run the application or script, whether or not a segment has output rows to process.

Use CREATE WRITABLE EXTERNAL WEB TABLE to define the external table and specify the application or script to run on the segment hosts. Commands run from within the database and cannot access environment variables (such as $PATH). Set environment variables in the EXECUTE clause of your writable external table definition. For example:

=# CREATE WRITABLE EXTERNAL WEB TABLE output (output text) 
    EXECUTE 'export PATH=$PATH:/home/`gpadmin`
            /programs;
    myprogram.sh' 
    FORMAT 'TEXT'
    DISTRIBUTED RANDOMLY;

The following Greenplum Database variables are available for use in OS commands run by a web or writable external table. Set these variables as environment variables in the shell that runs the command(s). They can be used to identify a set of requests made by an external table statement across the Greenplum Database array of hosts and segment instances.

Variable	Description
$GP_CID	Command count of the transaction running the external table statement.
$GP_DATABASE	The database in which the external table definition resides.
$GP_DATE	The date on which the external table command ran.
$GP_MASTER_HOST	The host name of the Greenplum master host from which the external table statement was dispatched.
$GP_MASTER_PORT	The port number of the Greenplum master instance from which the external table statement was dispatched.
$GP_QUERY_STRING	The SQL command (DML or SQL query) run by Greenplum Database.
$GP_SEG_DATADIR	The location of the data directory of the segment instance running the external table command.
$GP_SEG_PG_CONF	The location of the `postgresql.conf` file of the segment instance running the external table command.
$GP_SEG_PORT	The port number of the segment instance running the external table command.
$GP_SEGMENT_COUNT	The total number of primary segment instances in the Greenplum Database system.
$GP_SEGMENT_ID	The ID number of the segment instance running the external table command (same as `content` in `gp_segment_configuration`).
$GP_SESSION_ID	The database session identifier number associated with the external table statement.
$GP_SN	Serial number of the external table scan node in the query plan of the external table statement.
$GP_TIME	The time the external table command was run.
$GP_USER	The database user running the external table statement.
$GP_XID	The transaction ID of the external table statement.

Deactivating EXECUTE for Web or Writable External Tables

Parent topic: Unloading Data from Greenplum Database