ticdc: add a faq about generated column

[wk989898]

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• This PR is translated from: ticdc: add a faq about generated column docs#19369
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[qiancai]

Hi @wk989898, would you please involve tech reviewers for this PR? Thanks.

[qiancai]

Rest LGTM

[qiancai]

Suggested change

	## DML 中的生成列会发生什么行为？
	## TiCDC 会同步 DML 操作中的生成列吗？

[qiancai]

Suggested change

	生成列包括存储生成列和虚拟生成列。TICDC 会忽略虚拟生成列，而仅复制存储生成列到下游。当下游是 MySQL 数据库或其他与 MySQL 兼容的数据库，而不是 Kakfa 或其他存储服务时，存储生成列也会被忽略。
	生成列包括虚拟生成列和存储生成列。TiCDC 会忽略虚拟生成列，而仅同步存储生成列到下游。当下游是 MySQL 或其他与 MySQL 兼容的数据库，而不是 Kakfa 或其他存储服务时，存储生成列也会被忽略。

[qiancai]

Suggested change

	> **Note:**
	>
	> 当复制存储生成列到 Kafka 或存储服务后，再写回 MySQL 时，可能会发生错误 `Error 3105 (HY000): The value specified for generated column 'xx' in table 'xxx' is not allowed`。您可以使用[Open Protocol]（https://docs.pingcap.com/zh/tidb/stable/ticdc-open-protocol#ticdc-open-protocol）来避免此错误，因为该协议携带 column flag, 可以区分是否为生成列。
	> **注意：**
	>
	> 当同步存储生成列到 Kafka 或存储服务后，再将其写回 MySQL 时，可能会遇到 `Error 3105 (HY000): The value specified for generated column 'xx' in table 'xxx' is not allowed` 错误。为避免该错误，你可以使用 [Open Protocol](/ticdc/ticdc-open-protocol.md#ticdc-open-protocol) 进行同步。该协议的输出包含[列的 flag 值](/ticdc/ticdc-open-protocol.md#列标志位)，可以区分是否为生成列。

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-12-24 07:15:12.636273269 +0000 UTC m=+1545902.725075797: ☑️ agreed by qiancai.