Explain EXPLAIN (children 2)
 Set
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 2)
    ExpressionList (children 2)
     Identifier a.a2
     Identifier d.d2
    TablesInSelectQuery (children 4)
     TablesInSelectQueryElement (children 1)
      TableExpression (children 1)
       TableIdentifier a
     TablesInSelectQueryElement (children 2)
      TableExpression (children 1)
       TableIdentifier b
      TableJoin (children 1)
       ExpressionList (children 1)
        Identifier k
     TablesInSelectQueryElement (children 2)
      TableExpression (children 1)
       TableIdentifier c
      TableJoin (children 1)
       ExpressionList (children 1)
        Identifier k
     TablesInSelectQueryElement (children 2)
      TableExpression (children 1)
       TableIdentifier d
      TableJoin (children 1)
       ExpressionList (children 1)
        Identifier k
