title: Understanding spanner explanation
body: <p>I've a table of 860M rows in Google Cloud Spanner and I'm trying to understand how explanation works.</p>
  
  <p>The table has a string column <code>geoid</code> and there is an index at this column.</p>
  
  <p>When I run the following query it takes only 36ms:</p>
  
  <pre><code>SELECT count(*)
  FROM usbg_2015
  WHERE geoid= '340170175001'
  </code></pre>
  
  <p>Table structure is:</p>
  
  <pre><code>CREATE TABLE usbg_2015 (
      geoid STRING(12),
      quadkey STRING(24),
  ) PRIMARY KEY (geoid, quadkey)
  </code></pre>
  
  <p>However, I don't understand why the explanation says it uses a <strong>Table Scan</strong> instead of an <strong>Index Scan</strong>. I understood a <strong>Table scan</strong> as a full scan of the table, in this case reading 860M rows and it should take more time than 36ms. What I'm missing?</p>
  
  <p><a href="https://i.stack.imgur.com/pzFZr.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/pzFZr.png" alt="enter image description here"></a></p>