How to check if an array is empty in Postgres

Question

I have a Postgres function:

CREATE OR REPLACE FUNCTION get_stats(
    _start_date timestamp with time zone,
    _stop_date timestamp with time zone,
    id_clients integer[],
    OUT date timestamp with time zone,
    OUT profit,
    OUT cost
)
RETURNS SETOF record
LANGUAGE plpgsql
AS $$
DECLARE
    query varchar := '';
BEGIN
... -- lot of code
IF id_clients IS NOT NULL THEN
    query := query||' AND id = ANY ('||quote_nullable(id_clients)||')';
END IF;
... -- other code
END;
$$;

So if I run query something like this:

SELECT * FROM get_stats('2014-07-01 00:00:00Etc/GMT-3'
                      , '2014-08-06 23:59:59Etc/GMT-3', '{}');

Generated query has this condition:

"... AND id = ANY('{}')..."

But if an array is empty this condition should not be represented in query.
How can I check if the array of clients is not empty?

I've also tried two variants:

IF ARRAY_UPPER(id_clients) IS NOT NULL THEN
    query := query||' AND id = ANY ('||quote_nullable(id_clients)||')';
END IF;

And:

IF ARRAY_LENGTH(id_clients) THEN
    query := query||' AND id = ANY ('||quote_nullable(id_clients)||')';
END IF;

In both cases I got this error: ARRAY_UPPER(ARRAY_LENGTH) doesn't exists;

Answer 1

array_length() requires two parameters, the second being the dimension of the array:

array_length(id_clients, 1) > 0

So:

IF array_length(id_clients, 1) > 0 THEN
    query := query || format(' AND id = ANY(%L))', id_clients);
END IF;

This excludes both empty array and NULL.

Use cardinality() in Postgres 9.4 or later. See added answer by @bronzenose.

---

But if you're concatenating a query to run with EXECUTE, it would be smarter to pass values with a USING clause. Examples:

• Multirow subselect as parameter to `execute using`
• How to use EXECUTE FORMAT ... USING in Postgres function

---

To explicitly check whether an array is empty like your title says (but that's not what you need here) just compare it to an empty array:

id_clients = '{}'

That's all. You get:

true .. array is empty
null .. array is NULL
false .. any other case (array has elements - even if just null elements)