Oracle performance issue in getting first row in sub query

Question

I have a performance issue on the following (example) select statement that returns the first row using a sub query:

SELECT ITEM_NUMBER,
       PROJECT_NUMBER,       
       NVL((SELECT DISTINCT
                   FIRST_VALUE(L.LOCATION) OVER (ORDER BY L.SORT1, L.SORT2 DESC) LOCATION
            FROM   LOCATIONS L
            WHERE  L.ITEM_NUMBER=P.ITEM_NUMBER
                   AND L.PROJECT_NUMBER=P.PROJECT_NUMBER
           ),
          P.PROJECT_NUMBER) LOCATION
FROM   PROJECT P

The DISTINCT is causing the performance issue by performing a SORT and UNIQUE but I can't figure out an alternative.

I would however prefer something akin to the following but referencing within 2 select statements doesn't work:

SELECT ITEM_NUMBER,
       PROJECT_NUMBER,       
       NVL((SELECT LOCATION
            FROM   (SELECT L.LOCATION LOCATION
                           ROWNUM RN
                    FROM   LOCATIONS L
                    WHERE  L.ITEM_NUMBER=P.ITEM_NUMBER
                           AND L.PROJECT_NUMBER=P.PROJECT_NUMBER
                    ORDER BY L.SORT1, L.SORT2 DESC
                   ) R
            WHERE RN <=1
           ), P.PROJECT_NUMBER) LOCATION
FROM   PROJECT P

Additionally: - My permissions do not allow me to create a function. - I am cycling through 10k to 100k records in the main query. - The sub query could return 3 to 7 rows before limiting to 1 row.

Any assistance in improving the performance is appreciated.

Answer 1

It's difficult to understand without sample data and cardinalities, but does this get you what you want? A unique list of projects and items, with the first occurrence of a location?

SELECT 
P.ITEM_NUMBER,
P.PROJECT_NUMBER,       
MIN(L.LOCATION) KEEP (DENSE_RANK FIRST ORDER BY L.SORT1, L.SORT2 DESC) LOCATION
FROM   
LOCATIONS L
INNER JOIN
PROJECT P
ON L.ITEM_NUMBER=P.ITEM_NUMBER
AND L.PROJECT_NUMBER=P.PROJECT_NUMBER
GROUP BY
P.ITEM_NUMBER,
P.PROJECT_NUMBER

Answer 2

I encounter similar problem in the past -- and while this is not ultimate solution (in fact might just be a corner-cuts) -- Oracle query optimizer can be adjusted with the OPTIMIZER_MODE init param.

Have a look at chapter 11.2.1 on http://docs.oracle.com/cd/B28359_01/server.111/b28274/optimops.htm#i38318

FIRST_ROWS

The optimizer uses a mix of cost and heuristics to find a best plan for fast delivery of the first few rows. Note: Using heuristics sometimes leads the query optimizer to generate a plan with a cost that is significantly larger than the cost of a plan without applying the heuristic. FIRST_ROWS is available for backward compatibility and plan stability; use FIRST_ROWS_n instead.

Of course there are tons other factors you should analyse like your index, join efficiency, query plan etc..