INDEX SKIP SCAN適用場景

--請記住這個INDEX SKIP SCAN掃描方式


drop table t purge;
create table t as select * from dba_objects;
update t set object_type='TABLE' ;
commit;
update t set object_type='VIEW' where rownum<=30000;
commit;
create  index idx_type_id on t(object_type,object_id);
exec dbms_stats.gather_table_stats(ownname => 'LJB',tabname => 'T',estimate_percent => 10,method_opt=> 'for all indexed columns',cascade=>TRUE) ;  
set autotrace traceonly
set linesize 1000
select * from t where object_id=8;


執行計劃
-------------------------------------------------------------------------------------------
| Id  | Operation                   | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |             |     1 |    94 |     4   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T           |     1 |    94 |     4   (0)| 00:00:01 |
|*  2 |   INDEX SKIP SCAN           | IDX_TYPE_ID |     1 |       |     3   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------


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


   2 - access("OBJECT_ID"=8)
       filter("OBJECT_ID"=8)




統計資訊
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          7  consistent gets
          0  physical reads
          0  redo size
       1401  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

總結：
複合索引當謂詞條件不為索引的第一個欄位時，一般不會走索引，但是當複合索引的第一個欄位重複率較高時，會用到INDEX SKIP SCAN（記得收集統計資訊讓oracle知道列的分佈情況）。