title: How to achieve DISTINCT option in a tarantool query
body: <p>Articles are parsed from an rss feed and each article might fall into many categories.Also each article has some metadata such as source,upstream etc.</p>
  
  <p>Below is how we are designing the spaces.Each article is inserted into the articles space.  </p>
  
  <hr>
  
  <p><em>articles space</em></p>
  
  <p><strong>urlhash | article.content</strong><br>
  abcdef   | { dummy content}</p>
  
  <p>primary key urlhash = hash(article.url).  </p>
  
  <hr>
  
  <p>In the category_articles space we insert the article multiple times based on how many categories it falls into</p>
  
  <p><em>category_articles</em></p>
  
  <p><strong>source | category | urlhash | timestamp</strong><br>
  bbc      | arts     | article1  | 27777<br>
  bbc      | mobile   | article8  | 27777<br>
  bbc      | phone    | article3  | 27778<br>
  nyt      | sound    | article7  | 36667<br>
  nyt      | speaker  | article7  | 45556</p>
  
  <p>primary key = {source, category, urlhash}<br>
  secondary key = {source, category, timestamp}  </p>
  
  <p>I need latest articles  for a given source and a possible category.Below is how I framed the query.</p>
  
  <p>box.space.category_articles.index.secondary:select{{'nyt','speaker'},{ iterator = 'LE', limit = 5 }}</p>
  
  <p>Now I will get article7 twice in the result.Currently I am filtering duplicate results in the code.How can I have distinct(urlhash) kind of option in tarantool.</p>