注意，用于导出的外部表一定是Writable，赋权的时候一定要Grant ALL给一个用户，该用户才能使用这个外部表导出数据。

1. 带分隔符导出

可以只为外部表创建一个很长的字段，在select语句中拼接成一个字段：

CREATE Writable EXTERNAL TABLE schema_name.external_table_name
(
  exportdata character varying(3000)
)
 LOCATION (
    'gpfdist://192.168.0.240:10000/temp/my_table_file.txt'
)
 FORMAT 'text' (delimiter '^' null '' escape '\\')
ENCODING 'GB18030';
ALTER TABLE schema_name.external_table_name OWNER TO gpadmin;
GRANT ALL ON TABLE schema_name.external_table_name TO a_user;

2. 定长导出

drop external table schema_name.external_table_name;
CREATE Writable EXTERNAL TABLE schema_name.external_table_name
(
  field1 character varying(2),
  field2 character varying(8),
  field3 character varying(15),
  field4 character varying(1),
)
 LOCATION (
    'gpfdist://192.168.0.240:8080/export/my_fixedwith_file.txt'
)
FORMAT 'CUSTOM' (formatter='fixedwidth_out', field1='2', field2='8', field3='15', field4='1',  line_delim='\n' )
ENCODING 'GB18030';
GRANT ALL ON TABLE schema_name.external_table_name TO user_a;

建好后，通过客户端看到创建出的外部表定义如下：

CREATE EXTERNAL TABLE schema_name.external_table_name
(
  field1 character varying(2),
  field2 character varying(8),
  field3 character varying(15),
  field4 character varying(1),
)
 LOCATION (
     'gpfdist://192.168.0.240:8080/export/my_fixedwith_file.txt'
)
 FORMAT 'csv' (formatter 'fixedwidth_out' field1 '2' field2 '8' field3 '15' field4 '1' line_delim '
' )
ENCODING 'GB18030';
GRANT select ON TABLE schema_name.external_table_name TO user_a;

直接拿这个语句进行执行就会出现语法错误。