Oracle SELECT TOP 10 records

Question

Hi,

i have an big problem with an SQL Statement in Oracle. I want to select The TOP 10 Records ordered by STORAGE_DB wich arent in a list from an other select statement.

This one works fine for all records:

SELECT DISTINCT 
  APP_ID, 
  NAME, 
  STORAGE_GB, 
  HISTORY_CREATED, 
  TO_CHAR(HISTORY_DATE, 'DD.MM.YYYY') AS HISTORY_DATE  
  FROM HISTORY WHERE 
      STORAGE_GB IS NOT NULL AND 
        APP_ID NOT IN (SELECT APP_ID
                       FROM HISTORY
                        WHERE TO_CHAR(HISTORY_DATE, 'DD.MM.YYYY') = '06.02.2009') 

But when i am adding

AND ROWNUM <= 10
ORDER BY STORAGE_GB DESC

im getting some kind of "random" Records. I think because the limit takes in place before the order.

Does someone has an good solution? The other problem: This query is realy slow (10k+ records)

Answer 1

You'll need to put your current query in subquery as below :

SELECT * FROM (
  SELECT DISTINCT 
  APP_ID, 
  NAME, 
  STORAGE_GB, 
  HISTORY_CREATED, 
  TO_CHAR(HISTORY_DATE, 'DD.MM.YYYY') AS HISTORY_DATE  
  FROM HISTORY WHERE 
    STORAGE_GB IS NOT NULL AND 
      APP_ID NOT IN (SELECT APP_ID FROM HISTORY WHERE TO_CHAR(HISTORY_DATE, 'DD.MM.YYYY') ='06.02.2009')
  ORDER BY STORAGE_GB DESC )
WHERE ROWNUM <= 10

Oracle applies rownum to the result after it has been returned.
You need to filter the result after it has been returned, so a subquery is required. You can also use RANK() function to get Top-N results.

For performance try using NOT EXISTS in place of NOT IN. See this for more.

Answer 2

With regards to the poor performance there are any number of things it could be, and it really ought to be a separate question. However, there is one obvious thing that could be a problem:

WHERE TO_CHAR(HISTORY_DATE, 'DD.MM.YYYY') = '06.02.2009') 

If HISTORY_DATE really is a date column and if it has an index then this rewrite will perform better:

WHERE HISTORY_DATE = TO_DATE ('06.02.2009', 'DD.MM.YYYY')  

This is because a datatype conversion disables the use of a B-Tree ondex.

Answer 3

select * from (select empno,ename,sal,row_number() over(order by sal desc nulls last) rnm from emp) where rnm<=10