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

You will need to use self LEFT JOIN, like this (also on SQL Fiddle):

SELECT t.id, u.u
  FROM tab t
  LEFT JOIN (SELECT id, unnest(vals) u FROM tab) u
    USING (id);

Note, that for bigger tables query will be performing badly.

Answer 2

select id,
    unnest (
        "values"
        ||
        (array[null]::integer[])[1:(array_upper("values", 1) is null)::integer]
    )
from "table"

Answer 3

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

Answer 4

Upon revisiting this question it struck me that this can be simpler and faster.
Reverse the logic of the currently accepted solution by @a_horse:

SELECT id, CASE WHEN values <> '{}' THEN unnest(values) END AS value
FROM   tbl

• This returns a row with NULL in value for an empty array as well as for a NULL array, because only an array with elements in it produces TRUE in the test values <> '{}'.

• Works for arrays of any type, since the literal '{}' is automatically coerced to a matching type.

• Without explicit ELSE branch, CASE returns NULL, which is what we want anyway.

• Arrays with a NULL elements will return rows regardless.
  However. I found an anomaly there and posted a question concerning that:

  • NULL emements lost when casting result of unnest()

  Turned out to be a bug that was fixed after my report for pg 9.3+.

Answer 5

This works on Postgres 10 also:

SELECT id, UNNEST(CASE WHEN "values" <> '{}' THEN "values" ELSE '{null}' END)