title: Why does BigQuery do a full table scan for `SELECT *` with a WHERE clause referencing just one column?
body: <p>Why does BigQuery perform a full table scan for <code>SELECT *</code> when a WHERE clause is provided?</p>
  <pre><code>SELECT *
  FROM `githubarchive.month.202012` 
  WHERE login='__ThisUserDoesNotExist__'
  </code></pre>
  <p>This query performs a full table scan, even though it really just needs to do a full scan of the login column to determine that there are no records to return. Interested in references to relevant sections of BQ docs as well as papers on query planning for columnar databases.</p>