将Oracle CONNECT BY迁移到postgresql 10

Question

我有这样一个SQL：

SELECT count(*) as ct
          FROM classifications cls
WHERE
   cls.classification_id = :classification_id
START WITH cls.classification_id = :root_classification_id
CONNECT BY NOCYCLE PRIOR cls.classification_id = cls.parent_id

并需要将其迁移到PostgreSQL10。

我已经安装了扩展tablefunc并尝试使用connectedby。下面是我的尝试：

SELECT count(*) as ct
          FROM classifications cls
WHERE
   cls.classification_id = :classification_id

union
                  SELECT count(classification_id) FROM connectby('classifications','classification_id','parent_id',:root_classification_id,5)
                  as t(classification_id varchar, parent_id varchar,level int) 

问题是，联合是错误的方式，因为这样会得到计数的2个结果。

Answer 1

不需要使用tablefunc扩展名。这可以使用recursive CTE很容易地完成

with recursive tree as (

  select cls.classification_id, cls.parent_id 
  from classifications cls
  where cls.classification_id = :root_classification_id

  union all

  select child.classification_id, child.parent_id
  from classifications child
     join tree parent on child.parent_id = parent.classification_id
)
select count(*)
from tree;

CTE中的第一个查询与甲骨文的start with中的start with部件匹配。并且第二个查询中返回到CTE的联接与Oracle查询中的connect by部件相匹配。