不知道新版本的PG是否支持层次查询了。但可以考虑使用和构造递归查询，示例如下：

WITH recursive T (code , parent_code, name,h ) AS (
SELECT code , parent_code, name, name as h
FROM A
WHERE parent_code is null
UNION ALL
SELECT a.code , a.parent_code, a.name,t.h||','||a.name as h
FROM T , A
WHERE T.code = A.parent_code
)
SELECT * FROM T