How to force Dataset[] to output tables with named columns instead of named rows

Question

In the mathematica documentation it says that the syntax required to make "a table with named columns" is the following:

So consider some data arranged in this syntax:

dataset = Dataset[{
   <|"a" -> 1, "b" -> "x"|>,
   <|"a" -> 2, "c" -> "y"|>}
 ]

I want the "table with named columns" format above, but instead I keep getting the "table with named rows" format instead:

Question

How do I force mathematica to use the "table with named columns" format?

so that my output looks like so:

Please note that my actual table is more complicated and quite large. This is a minimalist example.

Answer 1

Take the KeyUnion of your Dataset