title: Substract 2 hstore, You might need to add explicit type casts
body: <p>Documentation for postgres has hstore - hstore which delete matching pairs from left operand. Installed postgres extensions.</p>
  
  <p>When I try</p>
  
  <pre><code>select public.hstore('"x"=&gt;"30", "y"=&gt;"c"') - 
         public.hstore('"x"=&gt;"30","y"=&gt;"fred"')
  </code></pre>
  
  <p>is error-ing with following </p>
  
  <pre><code>ERROR:  operator does not exist: public.hstore - public.hstore
  LINE 3:  select public.hstore('"x"=&gt;"30", "y"=&gt;"c"') - public.hstore...
                                                       ^
  HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.
  </code></pre>