title: insert with unnest function - skips a number in serial column
body: <p><strong>i am trying to use "unnest" function in insert,<br>
  while doing so, the serial skips a number for each insert,<br>
  plz help me to solve this...</strong></p>
  
  <pre><code>mydb=# \d tab1  
                           Table "public.tab1"  
   Column |  Type   |                     Modifiers                         
  --------+---------+---------------------------------------------------  
   id     | integer | not null default nextval('tab1_id_seq'::regclass)  
   col1   | integer |   
   col2   | integer |   
  Indexes:  
      "tab1_pkey" PRIMARY KEY, btree (id)  
  
  mydb=# insert into tab1(id,col1,col2) values (nextval('tab1_id_seq'),1,unnest(array[4,5]));  
  INSERT 0 2  
  mydb=# select * from tab1;  
   id | col1 | col2   
  ----+------+------  
    1 |    1 |    4  
    2 |    1 |    5  
  (2 rows)  
  
  mydb=# insert into tab1(id,col1,col2) values (nextval('tab1_id_seq'),2,unnest(array[4,5]));  
  INSERT 0 2  
  mydb=# select * from tab1;  
   id | col1 | col2   
  ----+------+------  
    1 |    1 |    4  
    2 |    1 |    5  
    4 |    2 |    4  
    5 |    2 |    5  
  (4 rows)  
  
  mydb=# insert into tab1(col1,col2) values(2,unnest(array[4,5]));  
  INSERT 0 2  
  mydb=# select * from tab1;  
   id | col1 | col2   
  ----+------+------  
    1 |    1 |    4  
    2 |    1 |    5  
    4 |    2 |    4  
    5 |    2 |    5  
    7 |    2 |    4  
    8 |    2 |    5  
  (6 rows)  
  
  mydb=# insert into tab1(col2) values(unnest(array[4,5]));  
  INSERT 0 2  
  mydb=# select * from tab1;  
   id | col1 | col2   
  ----+------+------  
    1 |    1 |    4  
    2 |    1 |    5  
    4 |    2 |    4  
    5 |    2 |    5  
    7 |    2 |    4  
    8 |    2 |    5  
   10 |      |    4  
   11 |      |    5  
  (8 rows)  
  
  mydb=# select nextval('tab1_id_seq');  
   nextval   
  ---------  
        13  
  (1 row)  
  </code></pre>
  
  <p><strong>for every insert it skips a number in id column,  help me solve this plz...</strong></p>