instance method fetch_primary_keys

# File activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb, line 542
        def fetch_primary_keys(tables)
          structures = table_structures(tables)

          tables.index_with do |table|
            _, structure = structure_for(structures, table)
            pks = structure.select { |f| f["pk"] > 0 }
            pks.sort_by { |f| f["pk"] }.map { |f| f["name"] }
          end
        end