select array[1,2,3]; 
return {1,2,3}

select array[1,2,3][1]; --会报错 
select (select array[1,2,3])[1] --可以使用

select regexp_match('123-123', '(\d+)-(\d+)'); --return {123, 123}
select regexp_match('123-123', '(\d+)-(\d+)')[1]; --报错

select(select regexp_match('123-123', '(\d+)-(\d+)'))[1]; --return 123

select 'a', array(select regexp_matches('aa-aa', '(aa)+', 'g'));
-- return 2 rows
a {aa}
a {aa}

select 'a', array(select regexp_matches('aa-aa', '(aa)+', 'g'));
--return 
a {{aa},{aa}}

select a, b[1][1] from (select 'a' as a, array(select regexp_matches('aa-aa', '(aa)+', 'g')) as b) as c;
--return 
aa

postgres=# select array_to_string (ARRAY[1,2,3],'##');
 array_to_string 
-----------------
 1##2##3
(1 row)
postgres=#