关于逻辑读问题

SQL> create table test(id int,name char(10));

Table created.

SQL> begin
  2  for i in 1 .. 10000
  3  loop
  4  insert into test values(i,'a'||i);
  5  end loop;
  6  commit;
  7  end ;
  8  /

PL/SQL procedure successfully completed.
SQL> select count(*) from test;

  COUNT(*)
----------
     10000

SQL> create index test_idx1 on test(id);

Index created.


BEGIN
  DBMS_STATS.GATHER_TABLE_STATS(ownname          => 'TEST',
                                tabname          => 'TEST',
                                estimate_percent => 100,
                                method_opt       => 'for all columns size skewonly',
                                no_invalidate    => FALSE,
                                degree           => 1,
                                cascade          => TRUE);
END;

SQL>  select count(distinct dbms_rowid.rowid_block_number(rowid)) from test;

COUNT(DISTINCTDBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID))
---------------------------------------------------
						 28


SQL> select count(distinct dbms_rowid.rowid_block_number(rowid)) from test where id<300;

COUNT(DISTINCTDBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID))
---------------------------------------------------
						  1

总共存放在28个块里,其中id<300的存放在一个块里

SQL> select * from test where id<300;

299 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2624864549

-----------------------------------------------------------------------------------------
| Id  | Operation		    | Name	| Rows	| Bytes | Cost (%CPU)| Time	|
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	    |		|   299 |  4485 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST	|   299 |  4485 |     3   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN	    | TEST_IDX1 |   299 |	|     2   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID"<300)


Statistics
----------------------------------------------------------
	  0  recursive calls
	  0  db block gets
	 43  consistent gets
	  0  physical reads
	  0  redo size
       8853  bytes sent via SQL*Net to client
	628  bytes received via SQL*Net from client
	 21  SQL*Net roundtrips to/from client
	  0  sorts (memory)
	  0  sorts (disk)

逻辑读是43 那43是怎么来的呢?
SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------

--------------------------------------------
SQL_ID	7vthtgk378xdk, child number 1
-------------------------------------
select * from test where id<300

Plan hash value: 2624864549

---------------------------------------------------------------------------------------------------
| Id  | Operation		    | Name	| Starts | E-Rows | A-Rows |   A-Time	| Buffers |
---------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	    |		|      1 |	  |    299 |00:00:00.01 |      43 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST	|      1 |    299 |    299 |00:00:00.01 |      43 |
|*  2 |   INDEX RANGE SCAN	    | TEST_IDX1 |      1 |    299 |    299 |00:00:00.01 |      22 |
---------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("ID"<300)


19 rows selected.

查看下索引的高度:
SQL> select index_name, PREFIX_LENGTH, BLEVEL, LEAF_BLOCKS from user_indexes where index_name='TEST_IDX1';

INDEX_NAME		       PREFIX_LENGTH	 BLEVEL LEAF_BLOCKS
------------------------------ ------------- ---------- -----------
TEST_IDX1					      1 	 21

可以看到索引高度为1,叶子节点有21个块

BLEVEL* NUMBER   B*-Tree level: depth of the index from its root block to its leaf blocks. A depth of 0 indicates that the root block and leaf block are the 

same.

最底层的索引块（叶块（leaf block））存储了被索引的数据值，以及对应的 rowid。一个leaf block存储多个被索引的数据值


因为ID<300的表数据都在一个块里,那么是不是回表就不会增加逻辑读,也就是说访问表的逻辑读始终为1,那么推测访问索引的逻辑读为42

那42是怎么来的呢？表示疑问？
从set autot trace看是索引读了1+21=22个,回表后逻辑读变为43 那回表都干了些什么呢 增加了21个逻辑读？21个逻辑读难道是正好回表21次,那不符合前面的逻辑？