title: Update in PostgreSQL performance relative to Select
body: <p>When I perform this simple Select query, the response is returned in 30ms.</p>
  <pre><code>select u.id, cv.id
  from units u
  left join volumes v on v.value = u.value
  left join carrier_units cu on cu.id = u.mccus_inspection_id
  left join carriers c on c.id = cu.carrier_id
  left join carrier_volumes cv on cv.vehicle_id = v.id and cv.carrier_id = c.id
  where u.carrier_vehicle_id is null
  and cv.id is not null
  and u.id = 115215784
  and cv.date = cu.date
  </code></pre>
  <p>However, when I try and perform an Update using an identical pattern, it takes 120 seconds or more.</p>
  <pre><code>UPDATE units
  SET carrier_vehicle_id = cv.id
  from units u
  left join volumes v on v.value = u.value
  left join carrier_units cu on cu.id = u.mccus_inspection_id
  left join carriers c on c.id = cu.carrier_id
  left join carrier_volumes cv on cv.vehicle_id = v.id and cv.carrier_id = c.id
  where u.carrier_vehicle_id is null
  and cv.id is not null
  and u.id = 115215784
  and cv.date = cu.date
  </code></pre>
  <p>How do I improve the performance of the Update statement to be more in line with the Select query?</p>