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I would recommend upgrading to start with, i had reported issues through the support just for searching on SKU.
For one we have: http://world.episerver.com/support/Bug-list/bug/COM-3315
As a workaround before that was released I created and registered my own ISearchQueryBuilder, you can implement that how you like to get whatever weighting you wish for.
Preferably inheriting from LuceneSearchQueryBuilder and override the BuildQuery method.
I like to give weight on SKU field while searching commerce catalog in CMS. In my case if I search by name like "canon" result pops-up however, if I search by SKU code then I do not see the relevent result.
So like to know if someway I can give more weight to certain fields. We are in commerce 9.16 and using standard cms indexing.