Re: How to do a Top-N analysis, while avoiding subqueries

Cannot see any problem with subquery i.e. why the second query is bad?
BTW both statistics are copied from the second run of each select to
avoid first time parsing overhead. Anyway to get the maximum count(*)
for each city you need to scan all table or probably all index if the
column is not null.

SQL> set arraysize 100
SQL> set autotrace on
SQL> create table temp as select * from all_objects;

Table created.

Elapsed: 00:00:03.05
SQL> select count(*) cnt, owner
 2  from temp group by owner
 3  order by cnt desc;

      CNT OWNER
---------- ------------------------------
    13893 SYS
    12287 PUBLIC
...
51 rows selected.

Elapsed: 00:00:00.03
Statistics
----------------------------------------------------------
         0  recursive calls
         0  db block gets
       805  consistent gets
         0  physical reads
         0  redo size
      2350  bytes sent via SQL*Net to client
       495  bytes received via SQL*Net from client
         2  SQL*Net roundtrips to/from client
         2  sorts (memory)
         0  sorts (disk)
        51  rows processed
SQL> ed
Wrote file afiedt.buf

 1  select * from (
 2  select count(*) cnt, owner
 3  from temp group by owner
 4  order by cnt desc
 5  )
 6* where rownum <=1
SQL> /

      CNT OWNER
---------- ------------------------------
    13893 SYS

Elapsed: 00:00:00.00
Statistics
----------------------------------------------------------
         0  recursive calls
         0  db block gets
       805  consistent gets
         0  physical reads
         0  redo size
       388  bytes sent via SQL*Net to client
       495  bytes received via SQL*Net from client
         2  SQL*Net roundtrips to/from client
         2  sorts (memory)
         0  sorts (disk)
         1  rows processed

SQL>

2006/6/7, tristan.van.essen@xxxxxxxxxxxxx <tristan.van.essen@xxxxxxxxxxxxx>:



Hi Muhammed



It's just a case study. Is it possible to do without the sub query?



Actually, the original question is not from me, but from a friend of mine. I
was just thinking about a good solution all morning long and couldn't figure
out a good answer to the question.



I find it quite reasonable to try a simple thing like a Top-N analysis
without the use of a (possibly costly) sub query.



But maybe I'm just spoiled by the proprietary MySQL feature of LIMIT BY…



Regards,

Tristan




------------------------------------------------------------------------------------

Tristan van Essen

Accenture Technology Solutions AG

Baslerstrasse 60

CH-8048 Zürich

Office:  +41 44 405 3245

Mobile: +41 79 654 0592

Email: tristan.van.essen@xxxxxxxxxxxxx
________________________________


From: Muhammed Soyer [mailto:msoyer@xxxxxxxxx] Sent: Mittwoch, 7. Juni 2006 12:02 To: van Essen, Tristan Cc: oracle-l@xxxxxxxxxxxxx Subject: Re: How to do a Top-N analysis, while avoiding subqueries




Why dont you want to use sub queries ? Any solid reason or just for curiosity ..





2006/6/7, tristan.van.essen@xxxxxxxxxxxxx <tristan.van.essen@xxxxxxxxxxxxx>:



Hi folks, this is my first question in this mailing list.



How do you do a top-n analysis while avoiding subqueries, but still use
group functions on other columns?



I have following Query:



SELECT COUNT(city) quantity, city

FROM customer

GROUP BY city

ORDER BY quantity DESC;

This works fine so far with the exception that I only want the first record
returned, instead of all. I tried to solve the problem with the ROWNUM
function, but it won't work well. The problem here is that I must include
ROWNUM in the GROUP BY clause, else the query fails. But then the result
isn't right anymore. I wish there was any sort of LIMIT BY clause, like
there is in MySQL.



How can I solve this, while not using any subqueries?



Regards,

Tristan





------------------------------------------------------------------------------------

Tristan van Essen

Accenture Technology Solutions AG

Baslerstrasse 60

CH-8048 Zürich

Office:  +41 44 405 3245

Mobile : +41 79 654 0592

Email: tristan.van.essen@xxxxxxxxxxxxx




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