Thank you for your information.
Here is the result:
After insertions:
ycsb=# select * from pgstatindex('usertable_pkey');
version | tree_level | index_size | root_block_no | internal_pages | leaf_pages | empty_pages | deleted_pages | avg_leaf_density | leaf_fragmentation
---------+------------+------------+---------------+----------------+------------+-------------+---------------+------------------+--------------------
2 | 3 | 5488721920 | 44337 | 4464 | 665545 | 0 | 0 | 52 | 11
(1 row)
After rebuild:
ycsb=# select * from pgstatindex('usertable_pkey');
version | tree_level | index_size | root_block_no | internal_pages | leaf_pages | empty_pages | deleted_pages | avg_leaf_density | leaf_fragmentation
---------+------------+------------+---------------+----------------+------------+-------------+---------------+------------------+--------------------
2 | 3 | 3154296832 | 41827 | 1899 | 383146 | 0 | 0 | 90.08 | 0
It seems like that rebuild has an effect to reduce the number of internal and leaf_pages and make more dense leaf pages.