ASA SQL User's Guide
Monitoring and Improving Performance
Top performance tips
Rebuilding your database is the process of unloading and reloading your entire database. It is sometimes called upgrading your database file format.
Rebuilding removes all the information, including data and schema, and puts it all back in a uniform fashion, thus filling space and improving performance much like defragmenting your disk drive. It also gives you the opportunity to change certain settings. In comparison, using the upgrade utility is quicker than rebuilding, and it does not affect the storage of your files.
When you upgrade your database, rebuilding also gives you access to all new features and performance enhancements in the latest version of the software.