title: Most efficient way to retrieve a unique list of keys from all rows of an hstore?
body: <p>For simplicity sake, say I have a table with a single column that is just an hstore. What is the most efficient way to go about getting a unqiue list of all the keys from all rows of the hstore?</p>
  
  <p>eg.</p>
  
  <pre><code>my_hstore
  ------------
  a=&gt;1,b=&gt;2
  b=&gt;2,c=&gt;3
  x=&gt;10,y=&gt;11
  y=&gt;11,z=12
  </code></pre>
  
  <p>What is the most efficient way to retrieve a list/array/set containing (a,b,c,x,y,z) ?</p>