title: how is backend_start great than xact_start
body: <p>How can the backend_start be greater than 2 days of xact_start/query_start? The 3rd sessions looks good, but the first 2 looks weird, is this possible? Would this mean anything?</p>
  <pre><code>pg=&gt; select * from pg_catalog.pg_stat_activity where usename = 'etl_user' and state = 'active' and backend_xmin = 65201266;              
  datid | datname |  pid  |usesysid | usename  |    application_name    |  client_addr   | client_hostname | client_port |         backend_start         |          xact_start           |         query_start          |         state_change          | wait_event_type | wait_event| state  | backend_xid | backend_xmin |                                                                                                    query                          |  backend_type
  -------+---------+-------+----------+----------+------------------------+----------------+-----------------+-------------+-------------------------------+-------------------------------+-------------------------------+-------------------------------+-----------------+------------+--------+-------------+--------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------
   16408 | pg| 37908 |   229661 | etl_user | PostgreSQL JDBC Driver |                |                 |             | 2021-04-20 21:36:22.540271+00 | 2021-04-17 22:31:32.314106+00 | 2021-04-17 22:31:32.317577+00 | 2021-04-20 21:36:22.541472+00 |                 || active |             |     65201266 | SELECT 1 FROM (SELECT ...)  | parallel worker
   16408 | pg| 37909 |   229661 | etl_user | PostgreSQL JDBC Driver |                |                 |             | 2021-04-20 21:36:22.540909+00 | 2021-04-17 22:31:32.314106+00 | 2021-04-17 22:31:32.317577+00 | 2021-04-20 21:36:22.542134+00 |                 || active |             |     65201266 | SELECT 1 FROM (SELECT ...)  | parallel worker
   16408 | pg|  3601 |   229661 | etl_user | PostgreSQL JDBC Driver | 10.175.130.142 |                 |       49832 | 2021-04-17 22:31:32.232008+00 | 2021-04-17 22:31:32.314106+00 | 2021-04-17 22:31:32.317577+00 | 2021-04-17 22:31:32.317578+00 |                 || active |             |     65201266 | SELECT 1 FROM (SELECT ...)  | client backend
  (3 rows)
  
  </code></pre>