Inserting array values

You should use a column name without an index to insert an array:

create table example(arr smallint[]);
insert into example(arr) values('{1, 2, 3}');
-- alternative syntax
-- insert into example(arr) values(array[1, 2, 3]);

select * from example;

   arr   
---------
 {1,2,3}
(1 row) 

Use the column name with an index to access a single element of the array:

select arr[2] as "arr[2]"
from example;

 arr[2] 
--------
      2
(1 row)

update example set arr[2] = 10;
select * from example;

   arr    
----------
 {1,10,3}
(1 row) 

You can use arr[n] in INSERT but this has special meaning. With this syntax you can create an array with one element indexed from the given number:

delete from example;
insert into example(arr[3]) values (1);
select * from example;

    arr    
-----------
 [3:3]={1}
(1 row) 

As a result you have an array which lower bound is 3:

select arr[3] from example;
 arr 
-----
   1
(1 row)