MERGE
  INTO NP D
  USING
  (
    SELECT ITEM,
           IND,
           TREE,
           NP_IRM,
           NP,
           NP_SOURCE
      FROM TEMP_NP
  ) S
  ON
  (
        D.ITEM = S.ITEM
    AND D.IND = S.IND
    AND D.TREE = S.TREE
    AND D.NP_IRM = S.NP_IRM
  )
  WHEN MATCHED THEN
    UPDATE
      SET D.NP_SOURCE = S.NP_SOURCE,
          D.NP = S.NP,
          D.UPDATE_DATETIME = SYSDATE
  WHEN NOT MATCHED THEN
    INSERT
    (
      D.ITEM,
      D.IND,
      D.TREE,
      D.NP_IRM,
      D.NP,
      D.NP_SOURCE,
      D.UPDATE_DATETIME
    )
    VALUES
    (
      S.ITEM,
      S.IND,
      S.TREE,
      S.NP_IRM,
      S.NP,
      S.NP_SOURCE,
      SYSDATE
    );

Explain plan

MERGE STATEMENT ()	[NULL]
 MERGE ()	NP
  VIEW ()	[NULL]
   HASH JOIN (OUTER)	[NULL]
    TABLE ACCESS (FULL)	TEMP_NP
    TABLE ACCESS (FULL)	NP

CREATE INDEX   IDX1_NET_PRICE
    ON   NET_PRICE(ITEM_SID)
TABLESPACE TEST_DATA
/

CREATE INDEX   IDX2_NET_PRICE
    ON   NET_PRICE(MAP_EFF_IRM)
TABLESPACE TEST_DATA
/

CREATE INDEX   IDX3_NET_PRICE
    ON   NET_PRICE(EST_METHOD_CODE)
TABLESPACE TEST_DATA
/

CREATE INDEX   IDX4_NET_PRICE
    ON   NET_PRICE(TREE_VERSION_SID)
TABLESPACE TEST_DATA
/

CREATE INDEX   IDX5_NET_PRICE
    ON   NET_PRICE(PPI_INDEX_SID)
TABLESPACE TEST_DATA
/

MERGE INTO NET_PRICE D USING ( SELECT ITEM_SID, PPI_INDEX_SID, 
  TREE_VERSION_SID, NET_PRICE_IRM, NET_PRICE, NET_PRICE_SOURCE FROM 
  TEMP_NET_PRICE ) S ON ( D.ITEM_SID = S.ITEM_SID AND D.PPI_INDEX_SID = 
  S.PPI_INDEX_SID AND D.TREE_VERSION_SID = S.TREE_VERSION_SID AND 
  D.NET_PRICE_IRM = S.NET_PRICE_IRM ) WHEN MATCHED THEN UPDATE SET 
  D.NET_PRICE_SOURCE = S.NET_PRICE_SOURCE, D.NET_PRICE = S.NET_PRICE, 
  D.UPDATE_DATETIME = SYSDATE WHEN NOT MATCHED THEN INSERT ( D.ITEM_SID, 
  D.PPI_INDEX_SID, D.TREE_VERSION_SID, D.NET_PRICE_IRM, D.NET_PRICE, 
  D.NET_PRICE_SOURCE, D.UPDATE_DATETIME ) VALUES ( S.ITEM_SID, 
  S.PPI_INDEX_SID, S.TREE_VERSION_SID, S.NET_PRICE_IRM, S.NET_PRICE, 
  S.NET_PRICE_SOURCE, SYSDATE )


call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          0          0           0
Execute      1     73.68     314.72     125768      40831    2044595      506757
Fetch        0      0.00       0.00          0          0          0           0
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        2     73.68     314.72     125768      40831    2044595      506757

Misses in library cache during parse: 0
Optimizer goal: CHOOSE
Parsing user id: 31     (recursive depth: 1)

Rows     Row Source Operation
-------  ---------------------------------------------------
      2  MERGE  
 506757   VIEW  
 506757    HASH JOIN OUTER 
 506757     TABLE ACCESS FULL TEMP_NET_PRICE 
4639556     TABLE ACCESS FULL NET_PRICE 

        D.ITEM   = S.ITEM
    AND D.IND    = S.IND
    AND D.TREE   = S.TREE
    AND D.NP_IRM = S.NP_IRM