If the table
being loaded has PRIMARY KEY and UNIQUE constraints, you can create the associated indexes
in parallel prior to enabling the constraints.
Bulk Data Moves: Using External Tables
You can query data from files outside the database via an object called an external table. An
external table??™s structure is defined via the organization external clause of the create table
command; its syntax closely resembles the SQL*Loader control file syntax.
You cannot manipulate rows in an external table, and you cannot index it??”every access of
the table results in a full table scan (that is, a full scan of the file at the operating system level). As
264 Oracle Database 11g DBA Handbook
a result, the performance of queries against external tables tends to be worse than that of tables
stored within the database. However, external tables offer a couple of potential benefits for
systems that load large sets of data:
Because the data is not stored within the database, the data is only stored once (outside
the database, rather than both outside and inside the database), thus saving space.
Because the data is never loaded into the database, the data-loading time is eliminated.
Given that you cannot index external tables, they are most useful for operations in which
large volumes of data are accessed by batch programs. For example, many data warehousing
environments have a staging area in which data is loaded into temporary tables prior to rows
being inserted into the tables users will query.
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