One should not reduce the size just because it is large per se. Size and performance are not related. Small models can (and often do) perform worse than large models because of the issues raised in my posts
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1. What is the Summery in line item? Summary is the built in feature in Anaplan (Blueprint mode) which tells the system if the numbers need be aggregated, averaged etc when the user navigates through hierarchy or time dimension.
2. Why summery line items consume high space? When you Keep summary as SUM, by default Anaplan understands that it has to aggregate the numbers and the cells where these numbers stay remains open and your number of cells increase - that's why you see the increase in size when you keep Summary on SUM
3. what points should consider before changing the summery? Only one point - whether or not you want to aggregate the numbers. If you don't want you can change the summary to None
4. I read your article regarding sparsity. Is it possible to apply the sparsity concept in Live models? If your Live Model is in Deployed state you anyway can't do that. But You can always optimize the model at any stage provided you know the model really well. It shouldn't happen that in order to optimize the model you actually take the functionality away from the business.
I think most of model optimization ways are already explained in previous messages. But in case of a multidimentional module were suppose you have more than 10 line items, where 5 line items are catering to 1 logic and another 5 to other logic than in that case some users create a line item in between those 5 line items just for reference purpose to segregate between two logics. Wherever such line items is used , you can make format to No Data , thus your cell count for that line item will zero and no space will be consumed. Hope it helps !!