Filtering first or constructing the hierarchical tree first?

A SQL with "start with" and "connect by" looks like:

select level, a.*
from t a
where a.ntid =2
start with a.vp_bid =9 or a.vp_bid is null
connect by prior a.bid= a.vp_bid;

My doubt is that Oracle will first apply the predicate: ntid=2 , or Oracle will
apply the "start with ... connect by" first, then do the filtiering? My test shows the latter is true


The rows in the table:

SQL> select * from t;

       BID       NTID     VP_BID
---------- ---------- ----------
         1          2
         2          2
         3          2          1
         4          2        100
         5          2          2
         6          7          2
         7          7          9
        10         10          9
        11         10         10
        12          2         10
        30         30         30

11 rows selected.

If filtering first, what we should see:

SQL> select level, a.* from
  2     ( select * from t where ntid=2) a
  3  start with a.vp_bid =9 or a.vp_bid is null
  4     connect by prior a.bid= a.vp_bid;

     LEVEL        BID       NTID     VP_BID
---------- ---------- ---------- ----------
         1          1          2
         2          3          2          1
         1          2          2
         2          5          2          2

If applying the hierarchical condition first, then the filtering, what we should see:

SQL> select * from (
  2     select level,  a.*
  3      from t a
  4      start with a.vp_bid =9 or a.vp_bid is null
  5     connect by prior a.bid= a.vp_bid
  6  )
  7  where ntid=2;

     LEVEL        BID       NTID     VP_BID
---------- ---------- ---------- ----------
         2         12          2         10
         1          1          2
         2          3          2          1
         1          2          2
         2          5          2          2

What we see from the original SQL confirms that Oracle applys the hierarchical condition first:

SQL> select level,  a.*
  2   from t a
  3   where a.ntid =2
  4   start with a.vp_bid =9 or a.vp_bid is null
  5  connect by prior a.bid= a.vp_bid;

     LEVEL        BID       NTID     VP_BID
---------- ---------- ---------- ----------
         2         12          2         10
         1          1          2
         2          3          2          1
         1          2          2
         2          5          2          2

Without the predicate, we should see:

SQL> select level,  a.*
  2       from t a
  3    start with a.vp_bid =9 or a.vp_bid is null
  4  connect by prior a.bid= a.vp_bid;

     LEVEL        BID       NTID     VP_BID
---------- ---------- ---------- ----------
         1          7          7          9
         1         10         10          9
         2         11         10         10
         2         12          2         10
         1          1          2
         2          3          2          1
         1          2          2
         2          5          2          2
         2          6          7          2

9 rows selected.