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Oracle Database Links Master Class

作者: Maclean Liu , post on December 12th, 2010 , English Version
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本文标题: Oracle Database Links Master Class
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Session is connected to a local instance and a database link is used to query data from one remote database instance.
The local instance sends the entire query to the remote database instance for processing
Query may access one or more tables, or views possibly using synonyms
The remote instance returns data to the local instance

Session is connected to a local instance and database links are used to query data from two or more database instances possibly including the local one
The query is always executed from the local database instance
The local instance decomposes the query into subqueries to be sent to each remote database instance.
The local instance retrieves data from remote instances and performs any necessary post-processing.

Session is connected to a local instance and a DB link is used to insert, update or delete data using one instance of a remote database.
The local instance sends the entire DML statement to the remote database instance for processing.
The statement may access one or more tables, or views possibly using synonyms
The remote instance returns DML results to the local instance
Single Phase Commit is used at transaction end

Session is connected to a local database instance and DB links used to perform DML on tables in two or more of the databases
Only one instance used for each remote database
Multiple DMLs may be sent to any of the databases involved in the transaction
The local instance retrieves the status from the remote instances for each DML
Two Phased Commit is used to Commit or Rollback the transaction guaranteeing consistency across all the databases involved in the transaction

DML and DDL statement can be executed in parallel, but with some restrictions:

All remote operations must be SELECT statements.
These statements must not be clauses in another distributed transaction.
A remote operation cannot be performed after issuing parallel DML/DDL or direct load INSERT.
DML is not parallelized if a distributed operation other than a SELECT is performed in the transaction.

See the Oracle8i Distributed Database Systems manual for more information.