Question

I am trying to use the unnest function in insert. While doing so, the serial skips a number for each insert.

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)  

For every insert it skips a number in the id column.

Answer 1

unnest returns multiple rows, so using it inside a single row of VALUES is a bit of a hack. Although it does work, it appears that the nextval call is somehow evaluated twice.

You can write an insert as INSERT INTO ... SELECT ... rather than INSERT INTO ... VALUES: in PostgreSQL, VALUES is just a row constructor. So consider writing something like this:

insert into tab1(col1, col2) select 1, unnest(array[4,5])