Question:  What does the impdp transform parameter do?  I read about thetransform=segment_attributes:n:table argument in the docs.

Answer:  Oracle data pump import (impdp) provides the transform=segment_attributes argument to allow you to change object DDL for specific objects (table, indexes), and remove just about everything from the DDL but the basic column definitions.

If the value is transform=segment_attributes:y (the default), then segment attributes (file attributes, storage parameters, tablespaces, and logging) are included in the import (impdp, with the DDL for the specified object types.

For example, if you wanted to use the default tablespace parameters for the imported DDL, you could include:

root>  impdp scott/tiger transform=segment_attributes:n:tables

This example would import only the "basic" DDL for all table DDL in the dmp file, omitting the storage clause, tablespace specification and datafiles parameters, allowing the system-wide defaults to be used.  This is useful when you don't want to have to pre-create all of the table definitions and import with ignore=y.

In another example, assume that you only wanted to omit the storage clauses for all tables:

root>  impdp scott/tiger transform=segment_attributes:storage:n

You can also use the impdp transform=segment_attributes:n is used with other arguments such as remap_tablespace  to change the tablespace names for all table in the import dmp file.  For example, the following directs the import (impdp) to ignore the existing tablespace specifications and re-map the tablespace names:

root>  impdp scott/tiger transform=segment_attributes:n remap_tablespace=users:prodts

You can also combine transform=segment_attributes parameters to perform multiple operations on the dmp file DDL. 

root>  impdp scott/tiger transform=SEGMENT_ATTRIBUTES:n:table,STORAGE:N:table