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Oracle® Database SQL Language Reference
11g Release 2 (11.2)

Part Number E17118-03
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Distributed Queries

The Oracle distributed database management system architecture lets you access data in remote databases using Oracle Net and an Oracle Database server. You can identify a remote table, view, or materialized view by appending @dblink to the end of its name. The dblink must be a complete or partial name for a database link to the database containing the remote table, view, or materialized view.

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Restrictions on Distributed Queries  Distributed queries are currently subject to the restriction that all tables locked by a FOR UPDATE clause and all tables with LONG columns selected by the query must be located on the same database. For example, the following statement raises an error because it selects press_release, a LONG value, from the print_media table on the remote database and locks the print_media table on the local database:

SELECT r.product_id, l.ad_id, r.press_release 
   FROM pm.print_media@remote r, pm.print_media l
   FOR UPDATE OF l.ad_id;

In addition, Oracle Database currently does not support distributed queries that select user-defined types or object REF data types on remote tables.