title: How to understand statistics of trace file in Oracle. Such as CPU, elapsed time, query...etc
body: <p>I am learning query optimization in Oracle and I know that trace file will create statistic about the query execution and EXPLAIN Plan of the query.</p>
  
  <p><img src="https://i.stack.imgur.com/l8058.png" alt="example trace file photo"></p>
  
  <p>At the bottom of the trace file, it is EXPLAIN PLAN of the query. My first question is , does the part "time = 136437 us" show the time duration for the steps of query execution? what does "us" mean ? Is it unit of time?</p>
  
  <p>In addition, can anyone explain what statistics such as count, cpu, elapsed ,  disk and query mean? I google and read Oracle doc about them already but I still can not understand it. Can anyone clarify the meaning of those stats more clearly?</p>
  
  <p>Thanks in advance.  I am new and sorry for my English. </p>