text: // When an index becomes multikey the query plan cache is cleared.  SERVER-5301
  
  t = db.jstests_queryoptimizer9;
  t.drop();
  
  function recordQueryPlan() {
      t.find( { a:1, b:1 } ).explain();
  }
  
  function hasCachedPlan() {
      return !!t.find( { a:1, b:1 } ).explain( true ).oldPlan;
  }
  
  t.ensureIndex( { a:1 } );
  t.save( { a:1 } );
  
  recordQueryPlan();
  assert( hasCachedPlan() );
  
  // While the index is not multikey the cached query plan is not cleared.
  t.save( { a:2 } );
  assert( hasCachedPlan() );
  
  // When the index becomes multikey the cached query plan is cleared.
  t.save( { a:[ 3, 4 ] } );
  assert( !hasCachedPlan() );
  
  // When the index remains multikey the cached query plan is not cleared.
  recordQueryPlan();
  t.save( { a:[ 5, 6 ] } );
  assert( hasCachedPlan() );
  