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。
