Add a checkbox to allow selecting only specific fields to be updated

[andykearney]

We use Feed Me with Google sheets pretty heavily for importing and updating products. After the initial import, further updates are usually fairly minimal. Best example being bulk updating ricing as it changes over time. In that case only a single field needs to be processed, along with the relevant fields for matching purposes. So 3 fields minimum in our case.

Having a check box for fields to be included next time the feed is run would be way more efficient than manually changing the field map each time.

Crude screenshot attached.

The end goal being to make updating a feed a lighter weight and therefore quicker task with less impact on CP and frontend performance.