訪問索引的方法

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1.B樹索引的結構

image

索引分支塊

      包含指向索引分支塊/葉子塊的指針和索引鍵值列

      （這裡指針指的是指向索引分支塊/葉子塊的塊地址RDBA。每個索引分支塊有兩種類型的指針，一種lmc（left most child）,一種索引行記錄所記錄的指針。lmc那就是指向最左邊的那個分支塊/葉子塊）

      lmc指向的分支塊/葉子塊的最大值一定<lmc在的那個分支塊的鍵值列的最小值

           簡單來說就是lmc那個地址>它下面最左邊的分支塊/葉子塊的地址

      索引行記錄所記錄的指針指向的分支塊/葉子塊的索引鍵值列的最小值一定>=該行記錄索引鍵值列的值

           例如索引行記錄AD指向的葉子塊都以AD開頭，所以AD<=AD開頭的葉子塊的值，相當於AD指向的葉子塊都是AD開頭

索引葉子塊

      索引鍵值和ROWID

2.常見訪問B樹索引的方法

————————————————————————————————————————————————————————

   （1）索引唯一性掃描（Index Unique Scan）

         針對Unique Index

         where條件是等值

         所以至多返回一條記錄

————————————————————————————————————————————————————————

    （2）索引範圍掃描（Index Range Scan）

          針對Unique Index時，where條件是between，> ,<等

          針對nonUnique Index時，可以=，>,<等

          可能返回多條記錄

—————————————————————————————————————————————————————————

     （3）索引全掃描（Index Full Scan）

            從左至右順序掃描全部葉子塊，單塊讀，不能並行

            因為索引有序，索引結果是按照鍵值列的順序排序。

            使用前提：索引鍵值列只要有一個是not null

SQL> set autotrace on

SQL> select empno ,ename from EMP;   --只有針對empno欄位的索引，這時候無法用到索引，走表全部掃描

     EMPNO ENAME
---------- ----------
      7369 SMITH
      7499 ALLEN
      7521 WARD
      7566 JONES
      7654 MARTIN
      7698 BLAKE
      7782 CLARK
      7788 SCOTT
      7839 KING
      7844 TURNER
      7876 ADAMS

     EMPNO ENAME
---------- ----------
      7900 JAMES
      7902 FORD
      7934 MILLER

14 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3956160932

--------------------------------------------------------------------------
| Id  | Operation      | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |     |    14 |   140 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP  |    14 |   140 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------


Statistics
----------------------------------------------------------
      1  recursive calls
      0  db block gets
      3  consistent gets
      0  physical reads
      0  redo size
    837  bytes sent via SQL*Net to client
    523  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
     14  rows processed

收集統計信息
SQL> exec dbms_stats.gather_table_stats(OWNNAME=>\'SCOTT\',tabname=>\'EMP\',estimate_percent=>100,cascade=>true,method_opt=>\'for all columns size 1\')
Index Full Scan實驗
實驗一：查詢有唯一索引的列
SQL> select empno from emp;

     EMPNO
----------
      7369
      7499
      7521
      7566
      7654
      7698
      7782
      7788
      7839
      7844
      7876

     EMPNO
----------
      7900
      7902
      7934

14 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 179099197

---------------------------------------------------------------------------
| Id  | Operation     | Name   | Rows  | Bytes | Cost (%CPU)| Time      |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT |      |    14 |    56 |    1   (0)| 00:00:01 |
|   1 |  INDEX FULL SCAN | PK_EMP |    14 |    56 |    1   (0)| 00:00:01 |
---------------------------------------------------------------------------


Statistics
----------------------------------------------------------
      1  recursive calls
      0  db block gets
      2  consistent gets
      0  physical reads
      0  redo size
    686  bytes sent via SQL*Net to client
    523  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
     14  rows processed
    
empno列上有唯一性索引PK_EMP，所以走的是Index Full Scan

實驗二：在允許為空的列上建索引
SQL> create index idx1_emp on emp(ename,deptno); 

Index created.

SQL> exec dbms_stats.gather_table_stats(OWNNAME=>\'SCOTT\',tabname=>\'EMP\',estimate_percent=>100,cascade=>true,method_opt=>\'for all columns size 1\')

PL/SQL procedure successfully completed.

SQL> select ename,deptno from emp;

ENAME           DEPTNO
---------- ----------
SMITH           20
ALLEN           30
WARD           30
JONES           20
MARTIN           30
BLAKE           30
CLARK           10
SCOTT           20
KING           10
TURNER           30
ADAMS           20

ENAME           DEPTNO
---------- ----------
JAMES           30
FORD           20
MILLER           10

14 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3956160932

--------------------------------------------------------------------------
| Id  | Operation      | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |     |    14 |   126 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP  |    14 |   126 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------


Statistics
----------------------------------------------------------
      1  recursive calls
      0  db block gets
      3  consistent gets
      0  physical reads
      0  redo size
    825  bytes sent via SQL*Net to client
    523  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
     14  rows processed

實驗結果：ename,deptno的鍵值都是允許為空，在上面列建索引，走TABLE ACCESS FULL

實驗三：修改一個列不允許為空
SQL> alter table emp modify(ename not null);  --把ename改為not null，走的是index full scan

Table altered.

SQL> exec dbms_stats.gather_table_stats(OWNNAME=>\'SCOTT\',tabname=>\'EMP\',estimate_percent=>100,cascade=>true,method_opt=>\'for all columns size 1\')

PL/SQL procedure successfully completed.

SQL> select ename,deptno from emp;

ENAME           DEPTNO
---------- ----------
ADAMS           20
ALLEN           30
BLAKE           30
CLARK           10
FORD           20
JAMES           30
JONES           20
KING           10
MARTIN           30
MILLER           10
SCOTT           20

ENAME           DEPTNO
---------- ----------
SMITH           20
TURNER           30
WARD           30

14 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 1165009684

-----------------------------------------------------------------------------
| Id  | Operation     | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT |        |     14 |    126 |      1   (0)| 00:00:01 |
|   1 |  INDEX FULL SCAN | IDX1_EMP |     14 |    126 |      1   (0)| 00:00:01 |
-----------------------------------------------------------------------------


Statistics
----------------------------------------------------------
      1  recursive calls
      0  db block gets
      2  consistent gets
      0  physical reads
      0  redo size
    825  bytes sent via SQL*Net to client
    523  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
     14  rows processed

實驗結果：走Index Full Scan

實驗四：在有很多null欄位的列上建索引

SQL> create index idx2_emp on emp(comm); 

Index created.

SQL> exec dbms_stats.gather_table_stats(OWNNAME=>\'SCOTT\',tabname=>\'EMP\',estimate_percent=>100,cascade=>true,method_opt=>\'for all columns size 1\')

PL/SQL procedure successfully completed.

SQL> select comm from emp;

      COMM
----------

       300
       500

      1400

 


     0


      COMM
----------

 


14 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3956160932

--------------------------------------------------------------------------
| Id  | Operation      | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |     |    14 |    28 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| EMP  |    14 |    28 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------


Statistics
----------------------------------------------------------
      1  recursive calls
      0  db block gets
      3  consistent gets
      0  physical reads
      0  redo size
    645  bytes sent via SQL*Net to client
    523  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
     14  rows processed
    
實驗結果：走TABLE ACCESS FULL    ，結果也體現出包含了那些null的結果

總結：在允許為null的列上建立索引，還是會走TABLE ACCESS FULL。因為欄位允許為null，而null並不會存儲索引的。只有走全錶掃描，才能體現值得完整性。

Index Full Scan掃描結果是有序的，順序按照索引鍵值列的順序
Index Full Scan必須滿足條件：目標索引至少有一個索引鍵值列的屬性為not null.

——————————————————————————————————————————————————————————
（4）索引快速全掃描（Index Fast Full Scan）
（1）僅適用于CBO
（2）使用多塊讀，可以並行
（3）結果不一定有序
其他和index full scan一樣咯
SQL> select /*+index_ffs(emp IDX1_EMP) */ename,deptno from emp;

ENAME           DEPTNO
---------- ----------
ADAMS           20
ALLEN           30
BLAKE           30
CLARK           10
FORD           20
JAMES           30
JONES           20
KING           10
MARTIN           30
MILLER           10
SCOTT           20

ENAME           DEPTNO
---------- ----------
SMITH           20
TURNER           30
WARD           30

14 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3993716296

---------------------------------------------------------------------------------
| Id  | Operation         | Name    | Rows    | Bytes | Cost (%CPU)| Time    |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |        |    14 |   126 |     2   (0)| 00:00:01 |
|   1 |  INDEX FAST FULL SCAN| IDX1_EMP |    14 |   126 |     2   (0)| 00:00:01 |
---------------------------------------------------------------------------------


Statistics
----------------------------------------------------------
      0  recursive calls
      0  db block gets
      4  consistent gets
      0  physical reads
      0  redo size
    825  bytes sent via SQL*Net to client
    523  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
     14  rows processed

————————————————————————————————————————————————————————

（5）索引跳躍掃描（Index Skip Scan）
簡單點來講就是你在ename,deptno兩個欄位建了索引，但是我們查詢的時候只用了deptno(沒有使用該索引的前導列)
但是僅僅使用于那些前導列的distinct值很少，後續非前導列選擇性又非常好的情景。
SQL> select ename from emp where deptno>20;

ENAME
----------
ALLEN
BLAKE
JAMES
MARTIN
TURNER
WARD

6 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3860167482

-----------------------------------------------------------------------------
| Id  | Operation     | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT |        |      7 |     63 |      1   (0)| 00:00:01 |
|*  1 |  INDEX SKIP SCAN | IDX1_EMP |      7 |     63 |      1   (0)| 00:00:01 |
-----------------------------------------------------------------------------

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

   1 - access("DEPTNO">20)
       filter("DEPTNO">20)


Statistics
----------------------------------------------------------
      1  recursive calls
      0  db block gets
      2  consistent gets
      0  physical reads
      0  redo size
    628  bytes sent via SQL*Net to client
    523  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
      6  rows processed

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