異構資料庫連線：PostgreSQL呼叫Oracle
oracle_fdw已經有較為正式的版本（oracle_fdw-1.1.0），分別試驗了本地PG連線到local windows oracle和本地PG連線到remote linux oracle（10.0.0.234）場景，select/insert/update/delete以及各種join/limit/order by等語句都可以很好的支援。
1、Install Oracle FDW
測試場景：PostgreSQL 9.3.4, Oracle XE 11g, oracle_fdw-1.1.0, Windows 7, Linux CentOS 6.3
https://github.com/laurenz/oracle_fdw/releases
把folder lib & share 裡的檔案copy 到PostgreSQL的安裝目錄
DROP EXTENSION oracle_fdw CASCADE;
CREATE EXTENSION oracle_fdw;
SELECT oracle_diag();
-- grant all on foreign data wrapper oracle_fdw to postgres;
2、Create connection
包括：install extension, external server name, user mapping, foregin table name
e.g.
CREATE SERVER oradb_234 FOREIGN DATA WRAPPER oracle_fdw OPTIONS (dbserver '//10.0.0.234:1521/devdb');
CREATE USER MAPPING FOR postgres SERVER oradb_234 OPTIONS (user 'test', password 'test');
CREATE FOREIGN TABLE ORA_234_TEST01 (id int options (key 'true'), name varchar(20)) SERVER oradb_234 OPTIONS (table 'TEST01'); -- TEST01 need to be uppercase
-- GRANT USAGE ON FOREIGN SERVER oradb_234 TO postgres;
SELECT rolname FROM pg_user -- pg_shadow, pg_roles
-- CREATE USER MAPPING FOR { username | USER | CURRENT_USER | PUBLIC }
3、Test connection
-- Remote Oracle
-- describe test01;
-- cmd>tnsping //10.0.0.234:1521/devdb-- expect OK
-- cmd>sqlplus test/test@//10.0.0.234:1521/devdb-- expect OK
-- PostgreSQL
-- \dx-- expect EXTENSION can be found
-- \d-- expect FOREIGN TABLE can be found
select * from ORA_234_TEST01;
insert into ORA_234_TEST01 values (2, 'from_PG');
update ORA_234_TEST01 SET name = 'from_PG1' where id = 2;
delete from ORA_234_TEST01 where id = 2;
4、Performance
效能方面當它需要copy大量資料時開銷較大，其它情況則還好。以下是一些例子：
select count(*) from ORA_234_TEST01; -- 184s, result: 971000
select * from ORA_234_TEST01 limit 1 offset 900000; -- 160s
select * from ORA_234_TEST01 limit 1; -- 0.1s
select * from ORA_234_TEST01 where id = 900001; -- 0.2s
5、Note
PostgreSQL 9.1 開始支援Oracle FDW
PostgreSQL 9.2 support for ANALYZE is available
PostgreSQL 9.3 Support for INSERT, UPDATE and DELETE is available
FOREIGN TABLE - OPTIONS: dbserver,user,password,table (required); schema,plan_costs,readonly (optional)
Remember that table and schema name must be in uppercase
update / delete: options (key 'true') is required
其它：還有一些FDW如JDBC_FDW, ODBC_FDW理論上也可以連去oracle，不過作者維護度很低也沒正式釋出版，不建議使用。
6、Reference
http://blog.itpub.net/133735/viewspace-730107
http://pgxn.org/dist/oracle_fdw/
http://francs3.blog.163.com/blog/static/4057672720122354546425/
http://www.csdn.net/article/2014-06-04/2820079/2
https://github.com/atris/JDBC_FDW
http://blog.osdba.net/522.html
http://www.postgresql.org/docs/9.3/static/fdwhandler.html
http://blog.163.com/digoal@126/blog/static/163877040201181505331588/