skip index scan

官网对skip index scan的解释：

Index skip scans improve index scans by nonprefix columns since it is often faster to scan index blocks than scanning table data blocks.

In this case a composite index is split logically into smaller subindexes. The number of logical subindexes depends on the cardinality of the initial column. Hence it is now possible to use the index even if the leading column is not used in a where clause.

从上面描述看，Oracle会对复合索引进行逻辑划分，分成多个子索引。逻辑子索引的个数依赖于前导列不同值个数，即使where子句中不使用前导列索引，仍有可能会使用到索引。

 

必须满足条件：

1、OPTIMIZER为CBO

2、Oracle 版本在9i以上

3、相关表和索引要进行过分析

 

skip scan会探测出索引前导列的唯一值个数，每个唯一值都会作为常规扫描的入口，在此基础上做一次查找，最后合并这些查询。例如：
表t1中有一个组合索引(owner,object_id)，其中owner只有test和sys两个值。在索引跳跃的情况下，我们可以逻辑上把他们看成两个索引，一个是（'TEST'，object_id)，一个是('SYS'，object_id).
select * from t1 where object_id=1;
发出这个查询后，oracle先进入owner为test的入口，查找object_id=1的条目。再进入owner为sys的入口，查找object_id=1的条目。最后合并两个结果集。

另外可以使用index_ss来强制选择skip index scan索引。

 

一个疑惑：

SQL> create table t1 as select * from dba_objects;

SQL> create index i_t1_owner on t1(owner,object_id);

SQL> exec dbms_stats.gather_table_stats(user,'t1');

 

SQL> select owner,count(*) from t1 group by owner;

OWNER                            COUNT(*)
------------------------------ ----------
PUBLIC                               3360
OUTLN                                   9
TEST                                    4
SYSTEM                                527
ORACLE_OCM                              8
SCOTT                                   6
DBSNMP                                 55
APPQOSSYS                               5
SYS                                  9094
WMSYS                                 318

已选择10行。

 

SQL> set autot trace

SQL> select count(*) from t1 where object_id=2 and owner>'TEST';

已用时间:  00: 00: 00.01

执行计划
----------------------------------------------------------
Plan hash value: 2159330979

-------------------------------------------------------------------------------
| Id  | Operation        | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |            |     1 |    10 |     3   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE  |            |     1 |    10 |            |          |
|*  2 |   INDEX SKIP SCAN| I_T1_OWNER |     1 |    10 |     3   (0)| 00:00:01 |
-------------------------------------------------------------------------------

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

2 - access("OWNER">'TEST' AND "OBJECT_ID"=2 AND "OWNER" IS NOT NULL)
       filter("OBJECT_ID"=2)

统计信息
----------------------------------------------------------
          5  recursive calls
          0  db block gets
         27  consistent gets
          0  physical reads
          0  redo size
        422  bytes sent via SQL*Net to client
        415  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

 

为什么这里明明使用了整个索引，为什么还是走了index skip scan呢？不是应该index range scan吗？

从谓词信息里看：

   2 - access("OWNER">'TEST' AND "OBJECT_ID"=2 AND "OWNER" IS NOT NULL)
       filter("OBJECT_ID"=2)

虽然扫描了整个索引块，但是却对索引块进行了 filter("OBJECT_ID"=2)过滤。