title: Using subquery for _PARTITIONTIME in bigquery does not limit cost
body: <p>When I run the below query on BQ using standard SQL, it says it will process 76.6TB when running</p>
  
  <pre><code>SELECT 
  event_time, user_id, activity_id,dbm_insertion_order_id, dbm_total_media_cost_usd 
  FROM `raw.5295.activity_*`
  WHERE _PARTITIONTIME &gt;(SELECT * FROM `analytics-dwh.autobidding.activity_list` )
  AND timestamp_micros(event_time)  &gt; (SELECT timestamp_micros(MAX(event_time)) from `essence-analytics-dwh.ml_for_autobidding.nest_na_4q18_activity_updated_daily`)
  AND _TABLE_SUFFIX IN ('25','20')
  </code></pre>
  
  <p>The table analytics-dwh.autobidding.activity_list has just one column with a unique list of integers</p>
  
  <p>If I remove the subquery from the above table, the query uses less than 500GB when run</p>
  
  <pre><code>SELECT 
  event_time, user_id, activity_id,dbm_insertion_order_id, dbm_total_media_cost_usd 
  FROM `raw.5295.activity_*`
  WHERE _PARTITIONTIME &gt;TIMESTAMP('2018-12-20')
  AND timestamp_micros(event_time)  &gt; (SELECT timestamp_micros(MAX(event_time)) from `essence-analytics-dwh.ml_for_autobidding.nest_na_4q18_activity_updated_daily`)
  AND _TABLE_SUFFIX IN ('25','20')
  </code></pre>
  
  <p>Why does this happen when I use a subquery? Is there a workaround?</p>