現在大家終于可以松下一口氣了，Oracle 開始較為詳細的說明這件事，并且將時間延遲到6月份，我以為原來的文章是內部留了一個余量，現在看來是修正了。我們此前也在文章中詳述了可選的解決方案，如果不啟用新的SCN兼容性3，補丁應用就不是必須的。

在文章中，首次澄清了補丁修正的內容，補充了以下詳細的描述，由于我們之前文章已經詳細解釋了這些內容，在此我只簡要的翻譯一下好了：

3. What is the change introduced by the patches listed above?

These patches increase the database's current maximum SCN (system change number) limit.

At any point in time, the Oracle Database calculates a "not to exceed" limit for the number of SCNs a database can have used, based on the number of seconds elapsed since 1988. This is known as the database's current maximum SCN limit. Doing this ensures that Oracle Databases will ration SCNs over time, allowing over 500 years of data processing for any Oracle Database.

These recommended patches enable the databases to allow for a higher current maximum SCN limit. The rate at which this limit is calculated can be referred to as the “SCN rate” and these patches help allow higher SCN rates to enable databases to support many times higher transaction rates than earlier releases.

Please note that the patches only increase the max limit but the current SCN is not impacted. So, if all your databases don’t have any major change in transaction rate, the current SCN would still remain below the current maximum SCN limit and database links between newer (or patched) and unpatched databases would continue to work. The patches provide the safety measure to ensure that you don’t have any issue with dblinks independent of any possible future change in your transaction rate.

請注意，這個補丁僅僅是增加了最大的SCN限制，所以如果你的數據庫在事務率方面沒有改變，或者事務率不高，當前的SCN將維持在最大SCN限制以下，在舊版本和新版本之間的DB link也可能毫無問題。

With the patches applied, this change in current maximum SCN limit will happen automatically starting 23rd June 2019.

如果應用了補丁，SCN 新算法的自動生效期是：2019年6月23日。

4. What happens if the recommended PSU / patchset is not applied?

If this patch is not applied, the unpatched database will have a lower SCN rate or lower current max SCN limit.

The newer or patched databases will have higher SCN rate or higher current max SCN limit.

如果不應用補丁，低版本的數據庫使用低SCN增長率，高版本數據庫使用高SCN增長率。這兩類數據庫互聯，就可能出現SCN的問題。

Therefore, there can be situations when the patched database is at a higher SCN level (due to the higher SCN rate allowance) and the unpatched database is at a much lower SCN level (due to lower SCN rate allowance).

When you open a dblink between these two databases, it has to sync the SCN levels of both the databases and if the SCN increase needed in the unpatched database for this sync is beyond it’s allowed SCN rate or current max SCN limit, then the dblink connection cannot be established.

This situation will not rise immediately after the change, but can potentially arise any time after?23rd June 2019.

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