In databases containing many indexes and records, index pages can consume a lot of space in 4D's memory cache. In version 6.5.x, when the cache is full and 4D needs additional space, the data in the cache is not directly unloaded anymore. Before executing this operation, the program checks the data currently in the cache to see if it can gain space by compacting the index pages. This alternative allows you to avoid reloading the data later.
Note: You can disable this feature by using the routine SET DATABASE PARAMETER with Selector = 4 (Index Compacting).