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The following command is one, cp /dev/null really_big.log is another, and echo > really_big.log is yet another. All of these commands overwrite a large file with nothing.

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The terms loading and unloading in the context of external tables can be confusing, so let s pause and make sure you understand these terms without any ambiguity When you deal with external tables, this is what these terms mean: Loading data means reading data from an external table and loading it into a regular Oracle table Oracle first reads the data stream from the files you specify Oracle will then convert the data from its external representation to an Oracle internal data type and pass it along to the external table interface Unloading data means reading data from a regular Oracle table and putting it into an external table Actually, you ll be loading table data into an external file In Oracle Database 10g, you can load and transform large volumes of data into platform-independent, Oracle proprietary flat files for data propagation or storage.

The ORACLE_DATAPUMP access driver can load as well extract data; that is, it can both load an external table from a flat file and extract data from a regular database table to an external flat file This external flat file data is written in a proprietary format, which only the ORACLE_DATAPUMP access driver can read You can then use this newly created file to create an external table in the same database or a different database When you load an Oracle table from an external table (data loading), you use the INSERT INTO SELECT clause, as shown in Listing 13-4 When you populate an external table using Oracle table data (data unloading), you use the CREATE TABLE AS SELECT clause (CTAS), as shown later in Listing 13-6.

Following are some of the benefits of being able to populate tables with external tables: Loading table data into flat files means that you can now store data or move it to different databases easily External tables help move large volumes of data across platforms, since they are platform-independent In data warehousing environments, there are many situations where you need to perform complex ETL jobs You can use SQL transformations to manipulate the data in the external tables before reloading them into the same or other databases..

Note that when you talk about writing to external tables, you are really referring to writing to an external file. You use a SELECT statement to extract table data to this operating system file. The ORACLE_DATAPUMP access driver writes data to this file in a binary Oracle-internal Data Pump format, and you can then use this file to load another external table in a different database.

This is the resulting directory listing after the file has been zeroed out:

The example in Listing 13-5 shows how you can create an external table and populate it with data from an external flat file using the ORACLE_DATAPUMP access driver rather than the ORACLE_LOADER driver. Listing 13-5. Creating an External Table with the ORACLE_DATAPUMP Access Driver SQL> CREATE TABLE test_xt( 3 product_id NUMBER(6), 4 warehouse_id NUMBER(3), 5 quantity_on_hand NUMBER(8)) 7 ORGANIZATION EXTERNAL( 9 TYPE ORACLE_DATAPUMP 10 DEFAULT DIRECTORY ext_data_dir 11 LOCATION ('test_xt.dmp')); Table created. SQL> To load data from this external table into an existing database table, you can use the INSERT INTO . . . SELECT clause, as shown earlier in Listing 13-4.