Why would this query cause a Merge Cartesian Join in Oracle

Question

I have a query that was recently required to be modified.

Here\'s the original

SELECT RTRIM (position) AS \"POSITION\",
   .  // Other fields
   .
           


        

Answer 1

The problem is, Oracle doesn't know that get_fiscal_year_start_date (SYSDATE) returns one single result. So it's assuming that it will generate lots of rows.

Obviously I don't have a test harness to hand, but this version of your query ought to banish the merge cartesian join.

SELECT RTRIM (position) AS "POSITION", 
.  // Other fields 
. 
. 
FROM schema.table x 
     , ( select get_fiscal_year_start_date (SYSDATE) as fiscal_year 
         from dual ) fy
WHERE hours > 0  
AND pay = 'RGW' 
AND NOT EXISTS( SELECT position 
                FROM  schema.table2 y 
                where y.date = fy.fiscal_year
                AND y.position = x.position ) 

Oracle knows that DUAL has a single row, and hence that the sub-query will return one value.