title: PostgreSQL gapless sequences
body: <p>I'm moving from MySql to Postgres, and I noticed that when you delete rows from MySql, the unique ids for those rows are re-used when you make new ones.  With Postgres, if you create rows, and delete them, the unique ids are not used again.</p>
  
  <p>Is there a reason for this behaviour in Postgres?  Can I make it act more like MySql in this case?</p>