primary_keys

# File activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb, line 358
        def primary_keys(table_name) # :nodoc:
          query_values(<<~SQL, "SCHEMA")
            SELECT a.attname
              FROM (
                     SELECT indrelid, indkey, generate_subscripts(indkey, 1) idx
                       FROM pg_index
                      WHERE indrelid = #{quote(quote_table_name(table_name))}::regclass
                        AND indisprimary
                   ) i
              JOIN pg_attribute a
                ON a.attrelid = i.indrelid
               AND a.attnum = i.indkey[i.idx]
             ORDER BY i.idx
          SQL
        end