The optimization process and recommendations:

1. Avoid Calling Functions With Indexed Columns (query line: 15): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `CLM_SERV_STRT` is indexed, the index won’t be used as it’s wrapped with the function `TRUNC`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
2. Avoid Calling Functions With Indexed Columns (query line: 16): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `CLM_SERV_STRT` is indexed, the index won’t be used as it’s wrapped with the function `TRUNC`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.
3. Avoid Calling Functions With Indexed Columns (query line: 17): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index. For example, if the column `CLM_COST_CTR_NBR` is indexed, the index won’t be used as it’s wrapped with the function `TRIM`. If you can’t find an alternative condition that won’t use a function call, a possible solution is to store the required value in a new indexed column.

The optimized query:

SELECT
        ItemA.CLM_SSN,
        ItemA.CLM_SERV_STRT Service_Date,
        ItemA.CLM_COST_CTR_NBR,
        ItemA.CLM_RECV_AMT,
        ItemB.CLM_COST_CTR_NBR RES_Cost_Center,
        ItemB.CLM_RECV_AMT,
        CST_SERV.GroupCode,
        Service 
    FROM
        DDIS.PTS_MV_CLM_STAT ItemA,
        DDIS.PTS_MV_CLM_STAT ItemB,
        DDIS.CST_SERV 
    WHERE
        TRUNC(ItemA.CLM_SERV_STRT) BETWEEN to_date('01-07-2013', 'dd-mm-yyyy') AND to_date('31-    07-2013', 'dd-mm-yyyy') 
        AND TRUNC(ItemA.CLM_SERV_STRT) = TRUNC(ItemB.CLM_SERV_STRT) 
        AND TRIM(ItemA.CLM_COST_CTR_NBR) = '5P311' 
        AND ITEMB.FK_SERV = CST_SERV.PKSERVICE 
        AND CST_SERV.GroupCode = 'RES' 
        AND Itema.CLM_SSN = ItemB.CLM_SSN 
        AND ItemA.CLM_RECV_AMT <> 0 
        AND ItemB.CLM_RECV_AMT <> 0 
    ORDER BY
        ItemA.CLM_SSN,
        ItemA.CLM_SERV_STRT