A write to the base table also writes to the GSI. If I project all the attributes, will it basically double the cost I currently pay for writes?
Also as an extra question, I have an LSI setup. That incurs costs on all my writes that I have that LSIs key in my items contents. If I don't include that key it will stop writing to the LSI right?
A GSI doubles the number of WCUs you are using (versus the base table only).
LSIs are 'free', and cost you no extra WCUs, as you only write to the base table.