Recently I found that WINDOW NOSORT STOPKEY with RANK()OVER() works very inefficiently: http://www.freelists.org/post/oracle-l/RANKWINDOW-NOSORT-STOPKEY-stopkey-doesnt-work
The root cause of this behaviour is that Oracle optimizes WINDOW NOSORT STOPKEY with RANK the same way as with DENSE_RANK:
[sourcecode language=”sql” highlight=””]
create table test(n not null) as
with gen as (select level n from dual connect by level<=100)
select g2.n as n
from gen g1, gen g2
where g1.n<=10
/
create index ix_test on test(n)
/
exec dbms_stats.gather_table_stats(”,’TEST’);
select/*+ gather_plan_statistics */ n
from (select rank()over(order by n) rnk
,n
from test)
where rnk<=3
/
select * from table(dbms_xplan.display_cursor(”,”,’allstats last’));
drop table test purge;
[/sourcecode]
[sourcecode language=”sql” highlight=”29,30,31″]
N
———-
1
1
1
1
1
1
1
1
1
1
10 rows selected.
PLAN_TABLE_OUTPUT
———————————————————————————————————————–
SQL_ID 8tbq95dpw0gw7, child number 0
————————————-
select/*+ gather_plan_statistics */ n from (select rank()over(order by
n) rnk ,n from test) where rnk<=3
Plan hash value: 1892911073
———————————————————————————————————————–
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
———————————————————————————————————————–
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.01 | 3 | | | |
|* 1 | VIEW | | 1 | 1000 | 10 |00:00:00.01 | 3 | | | |
|* 2 | WINDOW NOSORT STOPKEY| | 1 | 1000 | 30 |00:00:00.01 | 3 | 73728 | 73728 | |
| 3 | INDEX FULL SCAN | IX_TEST | 1 | 1000 | 31 |00:00:00.01 | 3 | | | |
———————————————————————————————————————–
Predicate Information (identified by operation id):
—————————————————
1 – filter("RNK"<=3)
2 – filter(RANK() OVER ( ORDER BY "N")<=3)
[/sourcecode]
As you can see, A-Rows in plan step 2 = 30 – ie, that is the number of rows where
DENSE_RANK<=3
but not
RANK<=3
The more effective way will be to stop after first 10 rows, because 11th row already has RANK more than 3!
But we can create own STOPKEY version with PL/SQL:
[sourcecode language="sql"]
create or replace type rowids_table is table of varchar2(18);
/
create or replace function get_rowids_by_rank(
n int
,max_rank int
)
return rowids_table pipelined
as
begin
for r in (
select/*+ index_rs_asc(t (n)) */ rowidtochar(rowid) chr_rowid, rank()over(order by n) rnk
from test t
where t.n > get_rowids_by_rank.n
order by n
)
loop
if r.rnk <= max_rank then
pipe row (r.chr_rowid);
else
exit;
end if;
end loop;
return;
end;
/
select/*+ leading(r t) use_nl(t) */
t.*
from table(get_rowids_by_rank(1, 3)) r
,test t
where t.rowid = chartorowid(r.column_value)
/
[/sourcecode]
