title: What does Oracle CONCURRENCY_WAIT_TIME mean here?
body: <p><strong>Question</strong></p>
  
  <ul>
  <li>For simple select query takes longer with concurrency_wait_time, while the accessing table does not get inserted, updated, or deleted, is there any way I can do some kind of <strong><em>dirty read</em></strong> for the row in the specific table in Oracle?</li>
  <li>What does CONCURRENCY_WAIT_TIME mean here exactly? Can I guess that concurrency_wait_time would occur when accessing a same row(record) on a table from multiple sessions? Even when using read only?</li>
  </ul>
  
  <p><strong>Situation</strong></p>
  
  <p>I've been doing load tests on WEB and REST API server, found simple select query was taking time longer than expected.</p>
  
  <p><strong>How it's tested</strong></p>
  
  <ul>
  <li>Jmeter from Desktop, 500 Concurrent POST request repeat 3 times and more</li>
  </ul>
  
  <p><strong>Throughput Result</strong></p>
  
  <ul>
  <li>30/sec when use 100 maxActive per instance</li>
  <li>22/sec when use 200 maxActive per instance</li>
  </ul>
  
  <p><strong>How to find slow query</strong></p>
  
  <ol>
  <li>Spring AspectJ on Application to write method process time by @around annotation</li>
  <li>Python script to analyze log to rank list methods with avg, min, max time</li>
  <li>Found some DB select calls take too long (2~20 secs) which usually takes less than a second.</li>
  <li>look up v$sqlarea to find slow query</li>
  <li><strong>Top 4 queries were found with average time 2076 ms, which had about 1970 ms concurrency_wait_time</strong></li>
  </ol>
  
  <p><a href="https://i.stack.imgur.com/vizpX.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vizpX.png" alt="enter image description here"></a></p>
  
  <blockquote>
    <p>CONCURRENCY_WAIT_TIME : Concurrency wait time (in microseconds)</p>
  </blockquote>
  
  <p><a href="https://docs.oracle.com/cd/B19306_01/server.102/b14237/dynviews_2129.htm#REFRN30259" rel="nofollow noreferrer">Oracle V$SQLAREA Definition</a></p>