What's the layout for DataMap fetching source data

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Hi,

We recently got an issue when executing datamap via smartpush from rules, which errored out during fetching source data because of "Unable to load the form as the number of cells exceeded the threshold 500000". We know the threshold and cut the smartpush into small chunks to avoid the limit at out best knowledge of the data. All works well till now and then it gets failed with the chunks setting. It's just surprising because based on digging into the data there should not be hitting the limits with the chunks. But there is one thing being the black box,