title: Materialized view log size > 0, when REFRESH is set as ON COMMIT
body: <p>I have a straight forward materialized view across about 8 master tables.</p>
  
  <pre><code>create materialized view MV
  REFRESH FAST ON COMMIT as
    SELECT...
  </code></pre>
  
  <p>I am also running this query to keep an eye on the materialized log sizes.</p>
  
  <pre><code>select segment_name, SUM ( (BYTES) / (1024 * 1024)) "Allocated(MB)" from dba_segments where (segment_name, owner) IN (
    select log_table, log_owner from dba_mview_logs where log_owner = 'XXX')
  and segment_type = 'TABLE' GROUP BY segment_name;
  </code></pre>
  
  <p>Since I am refreshing on commit, I wouldn't expect these logs to have an opportunity to grow. As soon as I write to the tables I expect the view to refresh and the log to clear.</p>
  
  <p>However, most of my logs are 0.0625mb with one at 27mb and another at 2mb.  How can I investigate the cause of the larger size?  Both of the tables with larger size logs containing blob columns holding possibly large data.  But I don't understand at a basic level why these logs are > 0.</p>