title: Postgres: on conflict with implicit conflict target
body: <p>Postgres 10 and 11 spec for insert says:</p>
  
  <blockquote>
    <p>ON CONFLICT [ conflict_target ] conflict_action </p>
  </blockquote>
  
  <p>I have a table:</p>
  
  <pre><code>create table c (
      e text not null,
      m text not null,
      v numeric not null,
      PRIMARY KEY (e, m)
  )
  </code></pre>
  
  <p>and I want to do</p>
  
  <pre><code>insert into candle values (...) 
      on conflict do update set
              v = 5
  </code></pre>
  
  <p>but I get an error:</p>
  
  <blockquote>
    <p>ON CONFLICT DO UPDATE requires inference specification or constraint name
      Hint: For example, ON CONFLICT (column_name)</p>
  </blockquote>
  
  <p>Why do I have to provide a conficting target? How to provide primary key or some other set of columns?</p>