Store select query's output in one array in postgres

I had exactly the same problem. Just one more working modification of the solution given by Denis (the type must be specified):

SELECT ARRAY(
SELECT column_name::text
FROM information_schema.columns
WHERE table_name='aean'
)

CREATE OR REPLACE FUNCTION f_test_array(in _colname text)
returns text as $body$
DECLARE colnames text[];
begin
colnames := ARRAY(
    SELECT column_name FROM information_schema.columns WHERE table_name='customer'
);
    if exists(select _colname = any(colnames))
    then return format('%s it exits.', _colname);
    else return format('%s not exits.', _colname);
end if;
end
$body$
LANGUAGE plpgsql;   

check if the column exists or not. Key point: if exists(select _colname = any(colnames)) We can also using string_agg String_agg usage:

CREATE OR REPLACE FUNCTION f_test_array1(in _colname text)
returns text as $body$
DECLARE colnames text;
begin
colnames := (SELECT string_agg(column_name,',') FROM information_schema.columns WHERE table_name='customer')::text;
if exists(select colnames ilike '%' || quote_literal(_colname) ||'%')
    then return format('column %s  exits.', _colname);
    else return format('column %s does not exits.', _colname);
end if;
end
$body$
LANGUAGE plpgsql;

Regular:

SELECT post_id FROM posts WHERE(poster_name='John');

output: [
  {'post_id': 1},
  {'post_id': 2},
  {'post_id': 3},
]

Using ARRAY_AGG:

SELECT ARRAY_AGG(post_id) FROM posts WHERE(poster_name='John');

output: [
 {[1, 2, 3]}
]

Casting to the datatype "TEXT" will ensure that your queries will run without any problem. In plpgsql when we assign to a array variable, we need not use the type casting. My requirement was to get a CSV of all the column names of a particular table. I'd used the following code in plpgsql.

Declare col_list varchar[]:=NULL;
cols varchar:=NULL;
Begin
    col_list := ARRAY(select t.name from frm_columns t where t.tname='emp_mstr');
    cols := array_to_string(col_list,',');
    return cols;
End;