title: Why do SQL id sequences go out of sync (specifically using Postgres)?
body: <p>I've seen solutions for updating a sequence when it goes out of sync with the primary key it's generating, but I don't understand how this problem occurs in the first place.  </p>
  
  <p>Does anyone have insight into how a primary key field, with its default defined as the nextval of a sequence, whose primary keys aren't set explicitly anywhere, can go out of sync with the sequence?  I'm using Postgres, and we see this occur now and then.  It results eventually in a duplicate key constraint when the sequence produces an id for an existing row.</p>