Sorting array elements - postgresql

Question

I want to write a stored procedure that gets an array as input parameter and sort that array and return the sorted array.

Answer 1

The best way to sort an array of integers is without a doubt to use the intarray extension, which will do it much, much, much faster than any SQL formulation:
CREATE EXTENSION intarray;
SELECT sort( ARRAY[4,3,2,1] );
A function that works for any array type is:
CREATE OR REPLACE FUNCTION array_sort (ANYARRAY)
RETURNS ANYARRAY LANGUAGE SQL
AS $$
SELECT ARRAY(SELECT unnest($1) ORDER BY 1)
$$;
(I've replaced my version with Pavel's slightly faster one after discussion elsewhere).

Answer 2

In PostrgreSQL 8.4 and up you can use:
select array_agg(x) from (select unnest(ARRAY[1,5,3,7,2]) AS x order by x) as _;
But it will not be very fast.
In older Postgres you can implement unnest like this
CREATE OR REPLACE FUNCTION unnest(anyarray)
RETURNS SETOF anyelement AS
$BODY$
SELECT $1[i] FROM
generate_series(array_lower($1,1),
array_upper($1,1)) i;
$BODY$
LANGUAGE 'sql' IMMUTABLE
And array_agg like this:
CREATE AGGREGATE array_agg (
sfunc = array_append,
basetype = anyelement,
stype = anyarray,
initcond = '{}'
);
But it will be even slower.
You can also implement any sorting algorithm in pl/pgsql or any other language you can plug in to postgres.

Answer 3

Just use the function unnest():
SELECT
unnest(ARRAY[1,2]) AS x
ORDER BY
x DESC;
See array functions in the Pg docs.

Answer 4

This worked for me from http://www.pgsql.cz/index.php/PostgreSQL_SQL_Tricks_I#General_array_sort
CREATE OR REPLACE FUNCTION array_sort (ANYARRAY)
RETURNS ANYARRAY LANGUAGE SQL
AS $$
SELECT ARRAY(
SELECT $1[s.i] AS "foo"
FROM
generate_series(array_lower($1,1), array_upper($1,1)) AS s(i)
ORDER BY foo
);
$$;
Please see Craig's answer since he is far more more knowledgable on Postgres and has a better answer. Also if possible vote to delete my answer.

Answer 5

Very nice exhibition of PostgreSQL's features is general procedure for sorting by David Fetter.
CREATE OR REPLACE FUNCTION array_sort (ANYARRAY)
RETURNS ANYARRAY LANGUAGE SQL
AS $$
SELECT ARRAY(
SELECT $1[s.i] AS "foo"
FROM
generate_series(array_lower($1,1), array_upper($1,1)) AS s(i)
ORDER BY foo
);
$$;

Answer 6

If you're looking for a solution which will work across any data-type, I'd recommend taking the approach laid out at YouLikeProgramming.com.
Essentially, you can create a stored procedure (code below) which performs the sorting for you, and all you need to do is pass your array to that procedure for it to be sorted appropriately.
I have also included an implementation which does not require the use of a stored procedure, if you're looking for your query to be a little more transportable.
Creating the stored procedure
DROP FUNCTION IF EXISTS array_sort(anyarray);
CREATE FUNCTION
array_sort(
array_vals_to_sort anyarray
)
RETURNS TABLE (
sorted_array anyarray
)
AS $BODY$
BEGIN
RETURN QUERY SELECT
ARRAY_AGG(val) AS sorted_array
FROM
(
SELECT
UNNEST(array_vals_to_sort) AS val
ORDER BY
val
) AS sorted_vals
;
END;
$BODY$
LANGUAGE plpgsql;
Sorting array values (works with any array data-type)
-- The following will return: {1,2,3,4}
SELECT ARRAY_SORT(ARRAY[4,3,2,1]);
-- The following will return: {in,is,it,on,up}
SELECT ARRAY_SORT(ARRAY['up','on','it','is','in']);
Sorting array values without a stored procedure
In the following query, simply replace ARRAY[4,3,2,1] with your array or query which returns an array:
WITH
sorted_vals AS (
SELECT
UNNEST(ARRAY[4,3,2,1]) AS val
ORDER BY
val
)
SELECT
ARRAY_AGG(val) AS sorted_array
FROM
sorted_vals
... or ...
SELECT
ARRAY_AGG(vals.val) AS sorted_arr
FROM (
SELECT
UNNEST(ARRAY[4,3,2,1]) AS val
ORDER BY
val
) AS vals

Answer 7

I'm surprised no-one has mentioned the containment operators:
select array[1,2,3] <# array[2,1,3] and array[1,2,3] #> array[2,1,3];
?column?
══════════
t
(1 row)
Notice that this requires that all elements of the arrays must be unique.
(If a contains b and b contains a, they must be the same if all elements are unique)