title: CREATE SCHEMA IF NOT EXISTS raises duplicate key error
body: <p>To give some context, the command is issued inside a task, and many task might issue the same command from multiple workers at the same time.</p>
  
  <p>Each tasks tries to create a postgres schema. I often get the following error:</p>
  
  <blockquote>
  <pre><code>IntegrityError: (IntegrityError) duplicate key value violates unique constraint "pg_namespace_nspname_index"
  DETAIL:  Key (nspname)=(9621584361) already exists.
   'CREATE SCHEMA IF NOT EXISTS "9621584361"'
  </code></pre>
  </blockquote>
  
  <p>Postgres version is PostgreSQL 9.4rc1.<br>
  Is it a bug in Postgres?</p>