title: group_concat query performance
body: <p>A MySQL query is running significantly slower since adding a <code>group_concat</code> clause. Currently, this query looks as follows:</p>
  
  <pre><code>select ... group_concat(distinct category.name) .... from page where 
  left outer join page_category on page.id = page_category.page_id  
  left outer join category on page_category.category_id = category.id
  ....
  group by page.id
  </code></pre>
  
  <p>As mentioned in the query, among others, my application has 3 tables: page, category, and page_category. A page can be associated with none or multiple categories. Currently page, page_category, and category have 9,460, 20,241 and 10 entries, respectively. </p>
  
  <p>Can anyone help me to improve this query to improve its performance?</p>