如何在分层查询中选择前N个元素（Oracle SQL）

Question

我有以下问题。在oracle12 db中有数百万个元素的hirarchy。如何选择前N个层次结构对象。应该选择哪个对象并不重要，唯一的要求是对象应该处于层次结构中。 Rownum过滤器根本不起作用：

SELECT object_id
FROM objects
WHERE rownum < 10
START WITH object_id = 9072243521013844211
CONNECT BY PRIOR object_id = parent_id

我得到以下执行计划：在对象选择后进行过滤。

/*
LEVEL   PLAN    ROWS    COST    PREDICATES
*/
1   SELECT STATEMENT (OPTIMIZER=ALL_ROWS)   8   1   

2     COUNT         

3       FILTER          filter(ROWNUM<10)

4         CONNECT BY (WITH FILTERING)           access("PARENT_ID"=PRIOR "OBJECT_ID")

5           TABLE ACCESS (BY INDEX ROWID) "OBJECTS"         

6             INDEX (UNIQUE SCAN) "XPKOBJECTS"  1   1   access("OBJECT_ID"=9072243521013844211)

5           NESTED LOOPS            

6             CONNECT BY PUMP           

6             TABLE ACCESS (BY INDEX ROWID BATCHED) "OBJECTS"   8   1   

7               INDEX (RANGE SCAN) "XIF25OBJECTS"   8   1   access("PARENT_ID"=PRIOR "OBJECT_ID")

Answer 1

以下示例解决方案应提供一些指导：

with objects as 
(select level object_id, decode(level+1, :len+1, null, level+1) parent_id 
from dual connect by level<=:len )
SELECT object_id
FROM objects
where level <= :n_objects
START WITH object_id = 1
CONNECT BY PRIOR parent_id = object_id
;