ergative case vs relative pronoun

ergative case

noun
  • A grammatical case used to indicate the agent of a transitive verb in ergative-absolutive languages. 

relative pronoun

noun
  • A pronoun that introduces a relative clause and refers to an antecedent. Some words that can be used as interrogative pronouns can alternatively be used as relative pronouns: what, which, who, and whom. The other relative pronouns are whoever, whomever, whatever, and that. 

How often have the words ergative case and relative pronoun occurred in a corpus of books? (source: Google Ngram Viewer )