2.3 What does purging Data mean?

What does purging Data mean?

1. In HP Vertica, delete operations do not remove rows from physical storage.
   The DELETE command in HP Vertica marks rows as deleted so that they remain available to historical queries
   
   Retention of historical data also applies to the UPDATE command, which is actually a combined DELETE and INSERT operation.

2. The cost of retaining deleted data in physical storage can be measured in terms of:

• Disk space for the deleted rows and delete markers
• A performance penalty for reading and skipping over deleted data

3. Purging : Permanently removes deleted data from physical storage so that the disk space can be reused.

4. We can control how much deleted data is retained in the physical storage used by your database by performing a purge operation using one of the following techniques:

• Setting a Purge Policy
   
• Manually Purging Data

Purpose of both these operations is to set the Ancient History Mark (AHM), which is an epoch that represents the time until which history is retained. History older than the AHM are eligible for purge.
AHM Is the epoch prior to which historical data can be purged from physical storage.