Starting in Oracle9i we got the ability to access non-Oracle files with Oracle SQL using external tables. Oracle external tables have important ramifications for systems where external files need to be available for non-database applications and appear to be a table within Oracle.

See these related notes on external tables:

• External Tables in Oracle RAC

• Oracle Spreadsheet SQL with External Tables

• Using Oracle External Tables for Data transfer

• Oracle External Table error and ORA-29913

• Oracle Disk I/O Tuning and External Tables

• Updating external tables in Oracle

• Oracle Table Design utl_file External Tables

• SQL*Loader External Tables

• Oracle SQL and External Tables

Inside Oracle external tables

Oracle external tables allow you to define the structure of almost any flat file on your server and have it appear to Oracle as if it were a real table:

 

Oracle read and write interfaces to OS files

As you can see, Oracle lets a database program write to flat files using the utl_file utility. Combined with Oracle external table’s read ability, this new topology removes the requirement that all Oracle data reside inside Oracle tables, opening new applications for Oracle. Let’s take a closer look at how this feature works.

Defining an external table


 
Now that we have created the directory for the external table, we can define the structure of the external file to Oracle.

Defining an Oracle external table

In this syntax, we define the column of the Oracle external table in much the same way as you would an internal Oracle table. The external definitions occur in the organization external clause.

Now that we’ve defined the Oracle external table, we can run reports against the external table using SQL, just as if the table resided inside the database. In the query shown in Listing B, note the use of the sophisticated ROLLUP parameter to summarize salaries by both department and job title. The results are available

Limitations on Oracle external tables

Because Oracle external tables were new to Oracle9i, Oracle had not yet perfected their use. In Oracle9i external tables have several limitations, including:
 

• No support for DML. Oracle external tables are read-only, but the base data can be edited in any text editor.
  
• Poor response for high-volume queries. Oracle external tables have a processing overhead and are not suitable for large tables.