Explain EXPLAIN (children 2)
 Set
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 3)
    ExpressionList (children 1)
     Function count (children 1)
      ExpressionList
    TablesInSelectQuery (children 4)
     TablesInSelectQueryElement (children 1)
      TableExpression (children 1)
       TableIdentifier a
     TablesInSelectQueryElement (children 2)
      TableExpression (children 1)
       TableIdentifier b
      TableJoin (children 1)
       Function equals (children 1)
        ExpressionList (children 2)
         Identifier b.b1
         Identifier a.a1
     TablesInSelectQueryElement (children 2)
      TableExpression (children 1)
       TableIdentifier c
      TableJoin (children 1)
       Function equals (children 1)
        ExpressionList (children 2)
         Identifier c.c1
         Identifier b.b1
     TablesInSelectQueryElement (children 2)
      TableExpression (children 1)
       TableIdentifier d
      TableJoin (children 1)
       Function equals (children 1)
        ExpressionList (children 2)
         Identifier d.d1
         Identifier c.c1
    ExpressionList (children 1)
     Identifier a.a2
