About Changes to External Tables in Greenplum 7

This topic describes the external table implementation and changes in Greenplum 7, and is geared towards existing users of Greenplum 6. Greenplum 7 converts an external table that you define with the CREATE EXTERNAL TABLE command into a foreign table, and internally operates on and represents the table using the foreign table data structures and catalog.

(See also Understanding the External Table to Foreign Table Mapping for detailed information about the external table to foreign table conversion, and its runtime implications.)

Parent topic: Accessing External Data with External Tables

What's the Same?

If you used external tables in Greenplum 6, the underlying functionality has not changed in Greenplum 7. The following external table features and behaviors remain the same in VMware Greenplum 7:

• Greenplum 7 fully supports the external table SQL command syntax of Greenplum 6.
• Greenplum 7 fully supports external table access to remote data sources via all existing protocols (file, gpdist, pxf, and s3).
• You must create separate tables to read from (CREATE EXTERNAL TABLE) and write to (CREATE WRITABLE EXTERNAL TABLE) the same external data location.
• The pg_exttable system catalog (now a view) provides the same information.

What Has Changed?

Note the following differences in the Greenplum 7 external table implementation compared to Greenplum 6:

• Greenplum 7 uses foreign table data structures and catalogs to internally represent external tables. Use the pg_foreign_table system catalog table and the ftoptions column to view the table definition.

• A pg_tables query no longer returns external tables in the query results.

• The pg_class.relkind of an external table is now f (was previously r).

• The pg_exttable system catalog is now a view.

• In addition to pg_exttable, you can use the following query to list all of the foreign tables that were created using the CREATE [WRITABLE] EXTERNAL TABLE command:

  SELECT * FROM pg_foreign_table ft 
    JOIN pg_foreign_server fs ON ft.ftserver = fs.oid
    WHERE srvname = 'gp_exttable_server';
  

• Because an external table is internally represented as a foreign table:

  • Every external table is associated with the gp_exttable_fdw foreign-data wrapper.
  • Every external table is associated with the gp_exttable_server foreign server.
  • Certain command output and error, detail, and notice messages about external tables refer to the table as a foreign table.
  • External tables are included in the foreign table catalogs, for example pg_foreign_table.
  • External tables are included when you list or examine foreign tables (for example, the \det psql meta-command).

• External table-specific information displayed in psql \dE+ output has changed; the relation Type of an external table is now foreign table. Example:

  \dE
                  List of relations
   Schema |     Name     |     Type      |  Owner  
  --------+--------------+---------------+---------
   public | ext_expenses | foreign table | gpadmin
  

• External table-specific information displayed in psql \d+ <external_table_name> output has changed; it now displays in foreign table format. For this example CREATE EXTERNAL TABLE call:

  CREATE EXTERNAL TABLE ext_expenses ( name text, date date, amount float4, category text, desc1 varchar )
    LOCATION ('gpfdist://etlhost-1:8081/*.txt', 'gpfdist://etlhost-2:8082/*.txt')
  FORMAT 'TEXT' ( DELIMITER '|' NULL ' ' )
  LOG ERRORS SEGMENT REJECT LIMIT 5;
  

  The example \d+ output follows:

  \d+ ext_expenses
                                           Foreign table "public.ext_expenses"
    Column  |       Type        | Collation | Nullable | Default | FDW options | Storage  | Stats target | Description 
  ----------+-------------------+-----------+----------+---------+-------------+----------+--------------+-------------
   name     | text              |           |          |         |             | extended |              | 
   date     | date              |           |          |         |             | plain    |              | 
   amount   | real              |           |          |         |             | plain    |              | 
   category | text              |           |          |         |             | extended |              | 
   desc1    | character varying |           |          |         |             | extended |              | 
  FDW options: (format 'text', delimiter '|', "null" ' ', escape E'\\', location_uris 'gpfdist://etlhost-1:8081/\*.txt|'gpfdist://etlhost-2:8082/\*.txt', execute_on 'ALL_SEGMENTS', reject_limit '5', reject_limit_type 'rows', log_errors 'enable', encoding 'UTF8', is_writable 'false')
  

• The EXPLAIN output for a query including an external table previously returned the text External Scan. EXPLAIN now returns Foreign Scan in this scenario.

Additional Considerations

Additional factors to consider:

• Even though an external table is internally represented as a foreign table, you cannot both read from and write to the same external table.
• You must change any scripts that you wrote that depend on external table DDL or psql \dE or \d+ output.
• Greenplum 7 dumps and restores the DDL of external tables using foreign table syntax.