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

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Given a keyword list containing search field, value and operator to search for (which may also be nested under logical operators, e.g., [{"id", {"franko", :==}}], or [{:and, [{"circle_id", {"123", :==}}, {"person_id", {"abc", :>}}]}), will return the dynamo db index description that will help us match this search. return :not_found if no index is found.

Returns a tuple of {"index_name", [ hash_key or hash,range_key]]} or :not_found TODO: Does not help with range queries. -> The match_index_hash_part function is beginning to address this.