参考： http://gpdb.docs.pivotal.io/4390/admin_guide/load/topics/g-working-with-file-based-ext-tables.html
- 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. - file:// Protocol
- gpfdist:// Protocol
- gpfdists:// Protocol
- gphdfs:// Protocol
- s3:// Protocol
- Using a Custom Protocol
- Handling Errors in External Table Data