Not the field names are indexed and stored, but the values.
So, no, all in all, you do not waste space. There is a slight overhead in
large _source fields if you mean that, but that is neglectable, when you
use compression, which is turned on by default.
You might save a few bytes, but end up with no hair after long debugging
session sometime in the future, trying to figure out what "gr" means and
what values does it hold
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