An index is a data directory that can quickly retrieve stored data, and what is stored in the index area of the data disk is the data directory. The data area of a data disk is a specific storage area of data.

When a new piece of data needs to be stored in the data disk in the database, an index will be established in the index area first, and then the specific data will be stored in the data area. This is the process of data storage. When deleting a piece of data, only the indexes in the index area will be deleted, but the data in the data area will not be deleted immediately, and the data in the data area without indexes will not be overwritten until the next new piece of data is stored. From this process, we can see that when we need to recover the deleted data, we only need to retrieve those data that have lost the index in the data area and re-index them. Of course, if the data is overwritten, it cannot be recovered.

These categories are clearly listed in the basic free video of Chanz Podcast. A video laid a solid foundation.