测试思路：
创建一个测试表t1，并创建索引，插入数据后，再将数据删除，然后对表和索引进行shrink操作，观察索引高度是否发生变化：

SQL> create table t1 as select * from dba_objects;              

Table created.

SQL> create index idx_t1 on t1(object_id,object_name) pctfree 90 tablespace idx_2k;

Index created.

SQL> analyze index idx_t1 validate structure;

Index analyzed.

SQL> select name,height,lf_blks,lf_rows,br_blks,br_rows from index_stats;

NAME                                                   HEIGHT    LF_BLKS    LF_ROWS    BR_BLKS    BR_ROWS
-------------------------------------------------- ---------- ---------- ---------- ---------- ----------
IDX_T1                                                      3       9552       9552         57       9551

SQL> select count(*) from t1;

  COUNT(*)
----------
      9552

SQL> delete from t1;

9552 rows deleted.

SQL> commit;

Commit complete.

SQL> analyze index idx_t1 validate structure;

Index analyzed.

SQL> select name,height,lf_blks,lf_rows,br_blks,br_rows from index_stats;

NAME                                                   HEIGHT    LF_BLKS    LF_ROWS    BR_BLKS    BR_ROWS
-------------------------------------------------- ---------- ---------- ---------- ---------- ----------
IDX_T1                                                      3       9552       1608         57       9551

SQL> alter table t1 enable row movement;

Table altered.

----不加cascade，只对表shrink，索引的高度，leaf block等不变。

SQL> alter table t1 shrink space;

Table altered.

SQL> analyze index idx_t1 validate structure;

Index analyzed.

SQL> select name,height,lf_blks,lf_rows,br_blks,br_rows from index_stats;

NAME                                                   HEIGHT    LF_BLKS    LF_ROWS    BR_BLKS    BR_ROWS
-------------------------------------------------- ---------- ---------- ---------- ---------- ----------
IDX_T1                                                      3       9552       1608         57       9551

----加cascade，对索引shrink，索引高度不变，leaf block，branch block减少，空间被回收，但维持高度不变

SQL> alter table t1 shrink space cascade;

Table altered.

SQL> analyze index idx_t1 validate structure;

Index analyzed.

SQL> select name,height,lf_blks,lf_rows,br_blks,br_rows from index_stats;

NAME                                                   HEIGHT    LF_BLKS    LF_ROWS    BR_BLKS    BR_ROWS
-------------------------------------------------- ---------- ---------- ---------- ---------- ----------
IDX_T1                                                      3          1          1          2          0

----rebuild索引，高度降低。

SQL> alter index idx_t1 rebuild;

Index altered.

SQL> analyze index idx_t1 validate structure;

Index analyzed.

SQL> select name,height,lf_blks,lf_rows,br_blks,br_rows from index_stats;

NAME                                                   HEIGHT    LF_BLKS    LF_ROWS    BR_BLKS    BR_ROWS
-------------------------------------------------- ---------- ---------- ---------- ---------- ----------
IDX_T1                                                      1          1          0          0          0

因此，相对于rebuild来说，在降低高水位的同时，shrink操作对索引的影响较小，没有降低索引高度，避免了root block分裂带来的风险。