title: How do I track changes to a variable in Postgresql database?
body: <p>I have a postgresql table which stores the inventories of several products in an ecommerce store. When customers buy (return) items, each item's inventory is decremented (incremented) to reflect the transaction. </p>
  
  <p>I am trying to write an analytics tool which shows when an item was changed and the quantity change - essentially a time series of that particular inventory item.</p>
  
  <p>Can someone please point me to a resource to help me?</p>
  
  <p>Thank you.</p>