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the best way to distinguish these is by the target for which the data is intended. As already mentioned, the target for ETL is a database. EAI, on the other hand, provides a framework for capturing data from one application and delivering it to another application, usually using some form of messaging technology. The target of EAI is an application. For example, EAI could capture new customers as they are added by the Customer Management application and send those to the Customer Service application so that the customer s services could be set up. In contrast, EII is a framework for delivering data from disparate sources to a user, generally in the form of query results. The target of EII is a user. Some EII tools provide agents that make disparate files look like relational databases to the user, who can then query them using SQL. While there is common confusion of the terms ETL, EAI, and EII, it is ETL that provides the capability to load (or reload) database tables on a scheduled basis.
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Several DBMS vendors have moved beyond their extract and load utility programs to offer support for table extraction within the DBMS itself. Oracle, for example, offers a materialized view facility (called snapshots prior to Oracle8i) to automatically create a local copy of a remote table. A materialized view is a view that actually stores the rows defined by the query included in the view definition. In its simplest form, the local table is a read-only replica of the remote master table that is loaded when the view is defined. However, materialized views can be defined so they are automatically refreshed by the Oracle DBMS on a periodic basis. Here is an Oracle SQL statement to create a local copy of product pricing data, assuming that the remote master database includes a PRODUCTS table like the one in the sample database: Create a local replica of pricing information from the remote PRODUCTS table.
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CREATE MATERIALIZED VIEW PRODPRICE AS SELECT MFR_ID, PRODUCT_ID, PRICE FROM PRODUCTS@REMOTE_LINK;
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This statement effectively creates a local Oracle table named PRODPRICE. It contains three columns, specified by the SELECT statement against the remote (master) database. The @ sign and name REMOTE_LINK in the statement tell Oracle that the PRODUCTS table from which the data is to be replicated is a remote table, accessible via the Oracle database link named REMOTE_LINK. The Oracle database administrator sets up these remote database links as part of the distributed Oracle capabilities they are required for access to a remote database using Oracle SQL. However, materialized views can be created based on local database objects as well as remote ones. In fact, this is commonly done in data warehouses and data marts, where materialized views are formed with summary information that is required for analysis. Finally, this CREATE MATERIALIZED VIEW statement will actually cause the local PRODPRICE materialized view to be populated with data from the remote PRODUCTS table and to be stored physically on the local database. With this type of read-only materialized view, users are not allowed to change the materialized view with INSERT, UPDATE, or DELETE statements. All database updates occur in the master (remote) table and are only propagated to the materialized view when it is refreshed. The database administrator can manually refresh the materialized view as desired using the DBMS_MVIEW.REFRESH stored procedure supplied by Oracle.
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The CREATE MATERIALIZED VIEW statement also includes rather comprehensive facilities for specifying automatic refreshes. Here are some examples: Create a local replica of pricing information from the remote PRODUCTS table. Refresh the data once per week, with a complete reload of the data.
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CREATE MATERIALIZED VIEW PRODPRICE REFRESH COMPLETE START WITH SYSDATE NEXT SYSDATE+7 AS SELECT MFR_ID, PRODUCT_ID, PRICE FROM PRODUCTS@REMOTE_LINK;
Create a local replica of pricing information from the remote PRODUCTS table. Refresh the data once per day, sending only changes from the master table.
CREATE MATERIALIZED VIEW PRODPRICE REFRESH FAST START WITH SYSDATE NEXT SYSDATE+1 AS SELECT MFR_ID, PRODUCT_ID, PRICE FROM PRODUCTS@REMOTE_LINK;
In the latter example, the FAST option specifies that the materialized view is refreshed by transmitting only changes from the remote PRODUCTS table. Oracle implements this capability by maintaining a log of changes on the remote system and updating the log every time an update to the PRODUCTS table would affect the materialized view. When the time for a refresh arrives, information from the change log is used. Complete refreshes can be expensive in terms of resources because the entire query must be run against the source table(s), sending all qualifying rows to the materialized view. For applications like this one, where product price changes probably affect only a small percentage of the overall table, this strategy is effective. The additional overhead of maintaining the log for the master table is more than offset by the reduced network traffic of transmitting only changed data. In other applications, where a large percentage of the rows in the master table will be modified between refreshes, it may be more efficient to simply do a complete refresh and eliminate the overhead of maintaining the log. By default, Oracle identifies rows (to determine whether they are changed) based on their primary key. If the primary key is not part of the replicated data, this can cause confusion about which rows have been updated; in this case, Oracle uses an internal row-id number (an option that can be specified when the materialized view is created) to identify the modified rows for refreshes to the materialized view. The SELECT statement that defines the materialized view offers a very general capability for data extraction. It can include a SELECT clause to extract only selected rows of the master table: Create a local replica of pricing information for high-priced products from the remote PRODUCTS table. Refresh the data once per day, sending only changes from the master table.
CREATE MATERIALIZED VIEW PRODPRICE REFRESH FAST START WITH SYSDATE NEXT SYSDATE+1 AS SELECT MFR_ID, PRODUCT_ID, PRICE FROM PRODUCTS@REMOTE_LINK WHERE PRICE > 1000.00;
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