Explain EXPLAIN QUERY TREE (children 2)
 Set
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 2)
    ExpressionList (children 7)
     Function sum (children 1)
      ExpressionList (children 1)
       Function plus (children 1)
        ExpressionList (children 2)
         Identifier a
         Literal UInt64_1
     Function sum (children 1)
      ExpressionList (children 1)
       Identifier b
     Function count (children 1)
      ExpressionList (children 1)
       Identifier b
     Function avg (children 1)
      ExpressionList (children 1)
       Identifier b
     Function count (children 1)
      ExpressionList (children 1)
       Function plus (children 1)
        ExpressionList (children 2)
         Identifier a
         Literal UInt64_1
     Function sum (children 1)
      ExpressionList (children 1)
       Function plus (children 1)
        ExpressionList (children 2)
         Identifier a
         Literal UInt64_2
     Function count (children 1)
      ExpressionList (children 1)
       Identifier a
    TablesInSelectQuery (children 1)
     TablesInSelectQueryElement (children 1)
      TableExpression (children 1)
       TableIdentifier fuse_tbl