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.

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

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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

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BTW, 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)

Answer 2

if for some reason you don't want to supply the dimension of the array, cardinality will return 0 for an empty array:

From the docs:

cardinality(anyarray) returns the total number of elements in the array, or 0 if the array is empty