You're seeing just the function get_matching_primary_index, go back to Ecto.Adapters.DynamoDB.Query module for more information.
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get_matching_primary_index(repo, tablename, search)

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Given a search criteria of 1 or more fields, we try find out if the primary key is a good match and can be used to forfill this search. Returns the tuple {:primary, [hash] | [hash, range]} or :not_found