A unique secondary key is implemented as a unique secondary index on the database. Its maintenance and creation have specific characteristics. The index must be kept current to ensure data integrity and performance, so it is updated synchronously whenever the table's data is modified (e.g., via INSERT, UPDATE, DELETE). This ensures that any subsequent read access using the index will find the correct data.

Furthermore, the ABAP Dictionary provides a flexible creation option. Instead of creating the index on the database immediately upon table activation, it can be defined for "lazy" creation. In this case, the physical index is only built on the database the first time a read access (e.g., a SELECT statement) attempts to use it.

A. It is created when a table is filled.

The index is created during table activation or upon first read access, a DDL-related event, not as a direct result of DML operations like filling the table.

B. It is updated when the modified table is read again.

Index maintenance is synchronous with data modification to ensure consistency; delaying updates until the next read would render the index inaccurate and unreliable.

1. SAP Help Portal

ABAP Keyword Documentation

ABAP Dictionary

Database Tables

Secondary Indexes:

"A secondary index is updated each time the table content is changed. This costs time." This statement supports option D.

"For this reason

you can also create secondary indexes in the ABAP Dictionary that are not created on the database immediately. The index is not created on the database until it is first accessed by a SELECT statement (or by a different read access)." This statement directly supports option C.

2. SAP Help Portal

ABAP Dictionary

Creating Secondary Indexes:

In the section on creating an index

the documentation describes the option for "Creation Type": "You can define whether the index must be created directly with the table in the database

or whether it can be created later when it is first used (that is

when the table is accessed for the first time using the index)." This confirms the "create on first access" feature (Option C). The entire purpose of an index necessitates it being updated upon modification (Option D)

which is a fundamental database principle also covered.