ASA Database Administration Guide
Backup and Data Recovery
Configuring your database for data protection
The size of the transaction log can determine what kind of backup is right for you, and can also affect recovery times.
You can control how fast the transaction log file grows by ensuring that all your tables have compact primary keys. If you carry out updates or deletes on tables that do not have a primary key or a unique index not allowing NULL, the entire contents of the affected rows are entered in the transaction log. If a primary key is defined, the database server needs to store only the primary key column values to uniquely identify a row. If the table contains many columns or wide columns, the transaction log pages fill up much faster if no primary key is defined. In addition to taking up disk space, this extra writing of data affects performance.
If a primary key does not exist, the engine looks for a UNIQUE NOT NULL index on the table (or a UNIQUE constraint). A UNIQUE index that allows NULL is not sufficient.