Vensterfuncties SORTEREN kostbaar, kunnen we het oplossen?

De prestaties van analytische functies kunnen afhankelijk zijn van de volgorde van de indexkolom. De index wijzigen van (ACCTNUM,DEPT_NUM) naar (DEPT_NUM,ACCTNUM) kan de kosten verlagen en de noodzaak voor tijdelijke tafelruimte wegnemen.

partition by COL_2 order by COL_1 => INDEX FAST FULL SCAN|WINDOW SORT PUSHED RANK
partition by COL_1 order by COL_2 => INDEX FULL SCAN|WINDOW NOSORT

INDEX FAST FULL SCAN gebruikt snellere multi-block IO, maar het vereist ook het sorteren van de gegevens en mogelijk tijdelijke tabelruimte voor het sorteergebied.

INDEX VOLLEDIGE SCAN gebruikt een langzamere single-block IO, maar het retourneert de gegevens in volgorde en vermijdt sorteren.

Voorbeeldschema en gegevens

--drop table mytable;
create table mytable(dept_num number not null, acctnum number not null
    ,a number, b number, c number, d number, e number);
insert into mytable
select 1 dept_num, 1 acctnum, 0,0,0,0,0 from dual union all
select 1 dept_num, 2 acctnum, 0,0,0,0,0 from dual union all
select 1 dept_num, 3 acctnum, 0,0,0,0,0 from dual union all
select 2 dept_num, 1 acctnum, 0,0,0,0,0 from dual union all
select 2 dept_num, 2 acctnum, 0,0,0,0,0 from dual union all
select 3 dept_num, 1 acctnum, 0,0,0,0,0 from dual;
--Create 600K similar rows.
insert into mytable
  select dept_num + rownumber*3, acctnum, a,b,c,d,e
  from mytable
  cross join (select level rownumber from dual connect by level <= 100000);
begin
    dbms_stats.gather_table_stats(user, 'mytable');
end;
/

(ACCTNUM,DEPT_NUM) =VENSTER SORTEER DE GEDRUKTE RANG

create index mytable_idx on mytable(acctnum, dept_num);

explain plan for
select dept_num, acctnum from
(
    select dept_num, acctnum
        ,row_number() over (partition by dept_num order by acctnum) as row_identifier
    from mytable
)
where row_identifier between 1 and 10;

select * from table(dbms_xplan.display);

Plan hash value: 952182109

------------------------------------------------------------------------------------------------
| Id  | Operation                | Name        | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |             |   600K|    22M|       |  1625   (3)| 00:00:23 |
|*  1 |  VIEW                    |             |   600K|    22M|       |  1625   (3)| 00:00:23 |
|*  2 |   WINDOW SORT PUSHED RANK|             |   600K|  4687K|  9424K|  1625   (3)| 00:00:23 |
|   3 |    INDEX FAST FULL SCAN  | MYTABLE_IDX |   600K|  4687K|       |   239   (3)| 00:00:04 |
------------------------------------------------------------------------------------------------

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

   1 - filter("ROW_IDENTIFIER">=1 AND "ROW_IDENTIFIER"<=10)
   2 - filter(ROW_NUMBER() OVER ( PARTITION BY "DEPT_NUM" ORDER BY "ACCTNUM")<=10)

(DEPT_NUM,ACCTNUM) =VENSTER NOSORT

drop index mytable_idx;
create index mytable_idx on mytable(dept_num, acctnum);

explain plan for
select dept_num, acctnum from
(
    select dept_num, acctnum
        ,row_number() over (partition by dept_num order by acctnum) as row_identifier
    from mytable
)
where row_identifier between 1 and 10;

select * from table(dbms_xplan.display);

Plan hash value: 1773829932

---------------------------------------------------------------------------------
| Id  | Operation         | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |             |   600K|    22M|   792   (2)| 00:00:12 |
|*  1 |  VIEW             |             |   600K|    22M|   792   (2)| 00:00:12 |
|*  2 |   WINDOW NOSORT   |             |   600K|  4687K|   792   (2)| 00:00:12 |
|   3 |    INDEX FULL SCAN| MYTABLE_IDX |   600K|  4687K|   792   (2)| 00:00:12 |
---------------------------------------------------------------------------------

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

   1 - filter("ROW_IDENTIFIER">=1 AND "ROW_IDENTIFIER"<=10)
   2 - filter(ROW_NUMBER() OVER ( PARTITION BY "DEPT_NUM" ORDER BY 
              "ACCTNUM")<=10)