title: sequence.nextval in oracle seems to increment twice
body: <p>I am trying to understand how nextval increments a sequence:
  in the following it seems that inside an INSERT INTO ...VALUES statement,
  nextval will increment twice the sequence. But only for the first such statement.
  I did not understand the reason for this behaviour in the <a href="http://docs.oracle.com/cd/B19306_01/server.102/b14200/pseudocolumns002.htm" rel="nofollow">ORACLE manual</a> </p>
  
  <pre><code>CREATE TABLE test_table(a INT);
  CREATE SEQUENCE seqa START WITH 10;
  CREATE SEQUENCE seqb START WITH 10;
  
  INSERT INTO test_table VALUES(seqb.NEXTVAL);
  INSERT INTO test_table VALUES(seqb.NEXTVAL);
  SELECT * FROM test_table;
  </code></pre>
  
  <p>Resulting test_table:</p>
  
  <pre><code>A
  -----
  11
  12
  </code></pre>
  
  <p>whereas:</p>
  
  <pre><code>SELECT seqa.NEXTVAL FROM DUAL; -- returns 10
  </code></pre>