RDBMS 12.2.0.1
CONFIGURE RETENTION POLICY TO RECOVERY WINDOW OF 7 DAYS;
之前，一直认为这句话的意思是保留7天（7天内）的备份。虽然之前在操作数据库的时候，发现了这样设置，7天前的备份，甚至10天前的备份仍旧没有被删除，当是比较纳闷，也没有多想，知道今天重新看了rman的文档，才发现之前自己的理解是错误的（可能和自己没上过任何的Oracle培训课程有关，自己理解有误）。
原文地址：（看了下面的官方解释，就明白了）
https://docs.oracle.com/en/database/oracle/oracle-database/12.2/bradv/rman-backup-concepts.html#GUID-A2C982AA-5385-4A45-B0D4-20C1CEFE3DED
8.9.1 About the Recovery Window
A recovery window is a period that begins with the current time and extends backward in time to the point of recoverability. The point of recoverability is the earliest time for a hypothetical point-in-time recovery, that is, the earliest point to which you can recover following a media failure.
For example, if you implement a recovery window of 1 week, then RMAN retains full backups and required incremental backups and archived logs so that the database can be recovered up to 7 days in the past. You implement this retention policy as follows:
CONFIGURE RETENTION POLICY TO RECOVERY WINDOW OF 7 DAYS;
This command ensures that for each data file, one backup that is older than the point of recoverability is retained. For example, if the recovery window is 7, then there must always exist one backup of each data file that satisfies the following condition:
SYSDATE - BACKUP CHECKPOINT TIME >= 7
All backups older than the most recent backup that satisfied this condition are obsolete.
Assume the retention policy illustrated in Figure 8-4. The retention policy has the following aspects:
The recovery window is 7 days.
Database backups are scheduled every two weeks on these days:
January 1
January 15
January 29
February 12
The database runs in ARCHIVELOG mode, and archived logs are saved on disk only if needed for the retention policy.
Figure 8-4 Recovery Window, Part 1
Description of "Figure 8-4 Recovery Window, Part 1"
As illustrated in Figure 8-4, the current time is January 23 and the point of recoverability is January 16. Hence, the January 15 backup is needed for recovery, and so are the archived logs from log sequence 500 through 850. The logs before 500 and the January 1 backup are obsolete because they are not needed for recovery to a point within the window.
Assume the same scenario a week later, as depicted in Figure 8-5.
Figure 8-5 Recovery Window, Part 2
Description of "Figure 8-5 Recovery Window, Part 2"
In this scenario, the current time is January 30 and the point of recoverability is January 23. Note how the January 15 backup is not obsolete even though a more recent backup (January 29) exists in the recovery window. This situation occurs because restoring the January 29 backup does not enable you to recover to the earliest time in the window, January 23. To ensure recoverability to any point in the window, you must save the January 15 backup and all archived logs from sequence 500 to 1150.