how to add extra field in 'order_item' table when product order success?

Question

Which event I have to use? I have need to store product wise shipping tax.

Answer 1

As per my understanding I think you have to use Magento extension attribute feature

If you want to learn how to use Magento Extension attribute then I think you can go through with this link :

https://store.fooman.co.nz/blog/an-introduction-to-extension-attributes.html

In your case in extension_attribute.xml of your module you have to add code like this, this is the sample of gift_message column at order item level

<extension_attributes for="Magento\Sales\Api\Data\OrderItemInterface">
<attribute code="gift_message" type="Magento\GiftMessage\Api\Data\MessageInterface" />
</extension_attributes>

Answer 2

I don't think you need to create an extra field on sales_order_item as magento already have tax fields to this table.

Answer 3

Magento already offers the tax field in it's table. Hence, I don't think you need to create an extra field. However, I can help you out if you can be a bit specific about what you wanna add.