可读外部表（不可以做dml操作）

可写外部表（只insert,不可以select,update,delete）

装载

创建外部表

=# CREATE EXTERNAL WEB TABLE ext_expenses (name text,
date date,amount float4,category text,description text)
LOCATION ( 'http://intranet.company.com/expenses/sales/file.csv',
'http://intranet.company.com/expenses/exec/file.csv',
'http://intranet.company.com/expenses/finance/file.csv',
'http://intranet.company.com/expenses/ops/file.csv',
'http://intranet.company.com/expenses/marketing/file.csv',
'http://intranet.company.com/expenses/eng/file.csv' )
FORMAT 'CSV' ( HEADER );

装载外部表数据

=# INSERT INTO expenses_travel
SELECT * from ext_expenses where category='travel';
或者想要快速装载全部数据到一个新的数据库表中：
=# CREATE TABLE expenses AS SELECT * from ext_expenses;

测试：

libo=# CREATE EXTERNAL WEB TABLE ext_expenses (name text)
libo-# location ('http://mirrors.aliyun.com/repo/Centos-6.repo')
libo-# FORMAT 'TEXT' ( DELIMITER '|' NULL ' ') ;
CREATE EXTERNAL TABLE

libo=# CREATE TABLE expenses AS SELECT * from ext_expenses;
NOTICE: Table doesn't have 'DISTRIBUTED BY' clause -- Using column(s) named 'colum' as the Greenplum Database data distribution key for this table.
HINT: The 'DISTRIBUTED BY' clause determines the distribution of data. Make sure column(s) chosen are the optimal data distribution key to minimize skew.
ERROR: could not translate host name "mirrors.aliyun.com",port "80" to address: Temporary failure in name resolution (cdbutil.c:754) (seg0 slice1 sdw1:40000 pid=26261) (cdbdisp.c:1489)
libo=#
libo=#
libo=# SELECT * from ext_expenses;
ERROR: could not translate host name "mirrors.aliyun.com",port "80" to address: Temporary failure in name resolution (cdbutil.c:754) (seg0 slice1 sdw1:40000 pid=26254) (cdbdisp.c:1489)

libo=# drop EXTERNAL WEB TABLE ext_expenses ;
DROP EXTERNAL TABLE
libo=# CREATE EXTERNAL WEB TABLE ext_expenses (colum text)
libo-# location ('http://115.28.122.210/repo/Centos-6.repo')
libo-# FORMAT 'TEXT' ( DELIMITER '|' NULL ' ') ;
CREATE EXTERNAL TABLE
libo=# select * from ext_expenses;
ERROR: connection with gpfdist Failed for http://115.28.122.210/repo/Centos-6.repo. effective url: http://115.28.122.210/repo/Centos-6.repo. (seg0 slice1 sdw1:40000 pid=26296)
libo=#

[root@mdw ~]# wget http://192.168.100.101:8081/aaa
--2014-03-04 14:14:01-- http://192.168.100.101:8081/aaa
正在连接 192.168.100.101:8081... 已连接。
已发出 HTTP 请求，正在等待回应... 200 ok
长度：未指定 [text/plain]
正在保存至: “aaa”

[ <=> ] 17 --.-K/s in 0s

2014-03-04 14:14:01 (1.61 MB/s) - “aaa” 已保存 [17]

libo=# CREATE EXTERNAL WEB TABLE ext_expenses (colum text)
libo-# location ('http://192.168.100.101:8081/aaa')
libo-# FORMAT 'TEXT' ( DELIMITER '|' NULL ' ') ;
CREATE EXTERNAL TABLE
libo=# select * from ext_expenses;
colum
-------
aaaa
aaa

aa
a


(7 rows)

create table t as select * from t_extDISTRIBUTED BY(id);

libo=# create table t as select * from t_ext DISTRIBUTED RANDOMLY;
SELECT 10

libo=# create external table t_ext (id int,name text)
libo-# location ('gpfdist://192.168.100.11:8081/aaa.csv')
libo-# format 'csv';
CREATE EXTERNAL TABLE
libo=# select * from t_ext;
ERROR: missing data for column "name" (seg3 slice1 sdw2:40001 pid=10243)
DETAIL: External table t_ext,line 4000 of gpfdist://192.168.100.11:8081/aaa.csv: ""

原因：csv 中有空行

结论：外部表只支持gpfdist 的http协议gpfdist服务是GP的简单的web服务

装载错误处理：

在定义可读外部表时使用CREATE EXTERNAL TABLE命令
结合使用SEGMENT REJECT LIMIT子句。
 拒绝限制count参数可用于指定记录数(缺省)，或者使用PERCENT指定记录
百分比。
 保存错误记录以备将来的检查，使用LOG ERRORS INTO子句指定错误记
录日志表。

使用gpload装载

卸载数据

禁止web表定义中使用execute

libo=# show gp_external_enable_exec
libo-# ;
gp_external_enable_exec
-------------------------
on
(1 row)

数据格式

在使用各种GP命令装载或卸载数据时，需要指定数据如何格式化

行分隔

GPDB预期是以LF字符(Line Feed/换行符/0x0A)、CR(Carriage Return/回车/0x0D)
或者CR加LF(CR+LF/回车换行/0x0A 0x0D)作为一行的分割。LF是标准UNIX或
类UNIX操作系统的标准换行标识符。其他操作系统(如Windows、Mac OS 9)可
能是CR或者CR+LF。所有这些换行标识符在GPDB中都被支持作为行分隔符

列分隔

对于TEXT文件来说缺省的列分隔符是TAB字符(0x09)，而 对 于CSV文件来说缺 省的列分隔符是逗号(0x2C)。不过在使用COPY、CREATE EXTERNAL TABLE 时或者使用gpload定义数据格式时都可以使用DELIMITER子句执行其他的单 字符分隔符。