PostgreSQL unnest with empty array

Question

I use postgreSQL 9.1. In my database there is a table which looks like

id | ... | values
-----------------------
1  | ... | {1,2,3}
2  | ... | {}

Answer 1

select id, 
       case 
         when int_values is null or array_length(int_values,1) is null then null
         else unnest(int_values)
       end as value
from the_table;

(note that I renamed the column values to int_values as values is a reserved word and should not be used as a column name).

SQLFiddle: http://sqlfiddle.com/#!1/a0bb4/1

---

Postgres 10 does not allow to use unnest() like that any more.

You need to use a lateral join:

select id, t.i
from the_table
   cross join lateral unnest(coalesce(nullif(int_values,'{}'),array[null::int])) as t(i);

Online example: http://rextester.com/ALNX23313

---

It can be simplified even further when using a left join instead of the cross join:

select id, t.i
from the_table
 left join lateral unnest(int_values) as t(i) on true;

Online example: http://rextester.com/VBO52351