【每日一摩斯】-Index Skip Scan Feature (212391.1)

INDEX Skip Scan，也就是索引快速扫描，一般是指谓词中不带复合索引第一列，但扫描索引块要快于扫描表的数据块，此时CBO会选择INDEX SS的方式。

官方讲的，这个概念也好理解，如果将复合索引看做是一个分区表，其中分区主键(这里指的是复合索引的首列)定义了存储于此的分区数据。在每个键(首列)下的每行数据都将按照此键排序。因此在SS，首列可以被跳过，非首列可以作为逻辑子索引访问。因此一个“正常”的索引访问可以忽略首列。

复合索引被逻辑地切分成更小的子索引。逻辑子索引的个数取决于初始列的cardinality。因此尽管首列未出现在谓词中，也可能使用这个索引。、

另外，需要吧补充一点：当复合索引的第一个字段的值重复率非常低时，扫描索引的效率会比全表扫描更高，这是CBO才可能会选择使用INDEX Skip Scan的方式访问数据。

这里比较奇怪的是：

使用9i时，未使用INDEX Skip Scan：

SQL> create table at2(a varchar2(3),b varchar2(10),c varchar2(5));
Table created.

SQL> begin
  2    for i in 1..1000
  3    loop
  4    insert into at2 values('M', i, 'M');
  5    insert into at2 values('F', i, 'F');
  6    end loop;
  7    end;
  8  /
PL/SQL procedure successfully completed.

SQL> create index at2_i on at2(a,b,c);
Index created.

SQL> exec dbms_stats.gather_table_stats(OWNNAME => NULL, TABNAME => 'at2', CASCADE => TRUE, method_opt => 'FOR ALL COLUMNS SIZE 1');
PL/SQL procedure successfully completed.



SQL> set autotrace traceonly



SQL> select * from at2 where b='352';
Execution Plan
----------------------------------------------------------
   0      SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=2 Bytes=14)
   1    0   INDEX (FAST FULL SCAN) OF 'AT2_I' (NON-UNIQUE) (Cost=2 Car
          d=2 Bytes=14)
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         10  consistent gets
          0  physical reads
          0  redo size
        447  bytes sent via SQL*Net to client
        587  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          2  rows processed

使用10g的，则使用了INDEX Skip Scan：

SQL> select * from full_tbl where object_name='TEST';
Execution Plan
----------------------------------------------------------
Plan hash value: 1293869270
------------------------------------------------------------------------------
| Id  | Operation         | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |          |     2 |    58 |    55   (2)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| FULL_TBL |     2 |    58 |    55   (2)| 00:00:01 |
------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("OBJECT_NAME"='TEST')

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