title: ArangoDB: what does execution cost represent on execution plan
body: <p>This is part of the execution plan that arango gave me.</p>
  
  <pre><code>Execution plan:
  Id   NodeType                  Est.   Comment
  40   CalculationNode            900           - LET now = DATE_NOW()   /* v8 expression */
  </code></pre>
  
  <p>As you can see, DATE_NOW() is costing 900.
  However, when I write a simplie query that only returns the value of DATE_NOW(), the execution cost is 1, just like below.</p>
  
  <pre><code>Execution plan:
   Id   NodeType          Est.   Comment
    1   SingletonNode        1   * ROOT
    2   CalculationNode      1     - LET #0 = DATE_NOW()   /* v8 expression */
    3   ReturnNode           1     - RETURN #0
  </code></pre>
  
  <p>I wish to know,<br>
  1. How does ArangoDB calculates the execution cost?<br>
  2. What does execution cost represents?</p>