SDB_ExtendRef - Extent reference

Extent references are instantiation objects, which define the root of an extent in an instance of a database object. The extent reference refers to one or more indexes, which provide different ways of accessing instances of the extent.

An extent can be marked as transient, i.e. extent data is not stored in the database. Depending on other settings, extent data can be stored in different places.

When the extent is referring to persistent instances, the collection for the extent (index) will be stored in a temporary main base, which is opened automatically, when opening the main base.

When the extent is referring to view instances, view instances are calculated when being accessed, when the extent is not marked as owning. To calculate instances only once, the view extent must be marked as owning. In this case, instances and indexes are stored in memory (view cache), when the create option is set to NO. Instances and indexes are stored in a temporary main base, when the create option is set to YES.

When transient is off (NO) for a view extent, view instances and indexes are stored to the database (persistent view).