Explain EXPLAIN QUERY TREE (children 2)
 Set
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 5)
    ExpressionList (children 1)
     Identifier key
    TablesInSelectQuery (children 1)
     TablesInSelectQueryElement (children 1)
      TableExpression (children 1)
       TableIdentifier test
    ExpressionList (children 1)
     Identifier key
    ExpressionList (children 2)
     OrderByElement (children 1)
      Function avg (children 1)
       ExpressionList (children 1)
        Identifier a
     OrderByElement (children 1)
      Identifier key
    Set
