After My RDS for PostgreSQL Instance Is Deleted, Why Can't the Associated Security Group Be Deleted Immediately?

When creating a DB instance, you must select a security group. If no security group is available or created, RDS allocates a security group to you by default.

After a DB instance is deleted, it is moved to the recycle bin and retained for seven days by default. To modify the retention period, configure a recycling policy.

The deleted instance is not removed from the security group immediately. The associated information is removed only after the instance is permanently deleted from the recycle bin. Before deleting a security group, ensure that the security group is not associated with any instance. For details about how to query instances associated with a security group, see How Do I Know the Instances Associated with a Security Group?