The second part, however, is what differentiates an external table from a regular table. This
is where the mapping between the database columns and the external data source occurs??”what
column(s) the data element starts in, how wide the column is, and whether the format of the
external column is character or binary. The syntax for the default type of external table, ORACLE_
LOADER, is virtually identical to that of a control file in SQL*Loader. This is one of the
advantages of external tables; the user only needs to know how to access a standard database
table to get to the external file.
There are a few drawbacks, however, to using external tables. You cannot create indexes on
an external table, and no inserts, updates, or deletes can be performed on external tables. These
drawbacks are minor when considering the advantages of using external tables for loading native
database tables, for example, in a data warehouse environment.
Chapter 1: Getting Started with the Oracle Architecture 13
Clustered Tables
If two or more tables are frequently accessed together (for example, an order table and a line item
detail table), then creating a clustered table might be a good way to boost the performance of
queries that reference those tables. In the case of an order table with an associated line-item detail
table, the order header information could be stored in the same block as the line-item detail
records, thus reducing the amount of I/O needed to retrieve the order and line-item information.
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