Stopkey Optimization – Part II

By | November 8, 2010

Continuing on my last post, I was curious to how the stopkey optimization is affected when using a between range instead of just an upper bound. This would be similar to queries that have pagination (e.g. page 1 is rows 1-100, page 2 is 101-200, etc.) Note that typically 100 rows per page is about 75 too many for an application like this, but that is a topic for another day…

I am using the same setup as the last post, just some new queries:

explain plan for
select
    small_vc
from
( 
    select  
        rownum rn, small_vc  
    from  
        t1  
    where  
        id > 100
)
where rn between 101 and 200;  

select * from table(dbms_xplan.display); 
----------------------------------------------------------------------------
| Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |  9901 |   193K|    48   (0)| 00:00:01 |
|*  1 |  VIEW               |      |  9901 |   193K|    48   (0)| 00:00:01 |
|   2 |   COUNT             |      |       |       |            |          |
|*  3 |    TABLE ACCESS FULL| T1   |  9901 |   145K|    48   (0)| 00:00:01 |
----------------------------------------------------------------------------

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

   1 - filter("RN"<=200 AND "RN">=101)
   3 - filter("ID">100)

So that didn’t work as hoped, as it didn’t do any stopkey optimization, but is doing a full table scan. We can force the stopkey optimization by putting the rownum filter in two steps:

explain plan for
select
    small_vc
from
( 
    select  
        rownum rn, small_vc  
    from  
        t1  
    where  
        id > 100
	and rownum < 200
)
where rn > 101; 

select * from table(dbms_xplan.display);
---------------------------------------------------------------------------------------
| Id  | Operation                     | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |       |   199 |  3980 |     6   (0)| 00:00:01 |
|*  1 |  VIEW                         |       |   199 |  3980 |     6   (0)| 00:00:01 |
|*  2 |   COUNT STOPKEY               |       |       |       |            |          |
|   3 |    TABLE ACCESS BY INDEX ROWID| T1    |   199 |  2985 |     6   (0)| 00:00:01 |
|*  4 |     INDEX RANGE SCAN          | T1_I1 |       |       |     2   (0)| 00:00:01 |
---------------------------------------------------------------------------------------

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

   1 - filter("RN">101)
   2 - filter(ROWNUM<200)
   4 - access("ID">100)

So that is a little uglier but does the optimization I was looking for. My last test is a twist on the first one, using the row_number() aggregate function instead of the normal rownum pseudocolumn:

explain plan for
select
    small_vc
from
( 
    select  
        row_number() over (order by 1) rn, small_vc  
    from  
        t1  
    where  
        id > 100
)
where rn between 101 and 200;  


select * from table(dbms_xplan.display);
----------------------------------------------------------------------------------------
| Id  | Operation                      | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT               |       |   199 |  3980 |     6   (0)| 00:00:01 |
|*  1 |  VIEW                          |       |   199 |  3980 |     6   (0)| 00:00:01 |
|*  2 |   WINDOW NOSORT STOPKEY        |       |   199 |  2985 |     6   (0)| 00:00:01 |
|*  3 |    COUNT STOPKEY               |       |       |       |            |          |
|   4 |     TABLE ACCESS BY INDEX ROWID| T1    |   199 |  2985 |     6   (0)| 00:00:01 |
|*  5 |      INDEX RANGE SCAN          | T1_I1 |       |       |     2   (0)| 00:00:01 |
----------------------------------------------------------------------------------------

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

   1 - filter("RN">=101 AND "RN"<=200)
   2 - filter(ROW_NUMBER() OVER ( ORDER BY  NULL )<=200)
   3 - filter(ROWNUM<200)
   5 - access("ID">100)

Definitely not what I expected, but interesting to note.

2 thoughts on “Stopkey Optimization – Part II

  1. Szalonooki

    last try should be,I guess:

    explain plan for
    select
    small_vc
    from
    (
    select
    row_number() over (order by 1) rn, small_vc
    from
    t1
    where
    id > 100
    and rownum 101;

    select * from table(dbms_xplan.display);

    Reply
  2. cmartin2 Post author

    @Szalonooki – Your example doesn’t look like it came through quite right.. can you try again?

    Reply

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