Update Multiple Column with Single Command with Join between Two tables.

Question

This is our command where we are trying to update a table's two columns by pulling values from another table after joining. This query takes too much time to execute. Any suggestions ?
UPDATE raw_alc_rmfscombinednew
SET (BSC, CELL_NAME) =
SELECT distinct raw_alc_R110combinednew.BSC, raw_alc_R110combinednew.CELL_NAME
FROM raw_alc_R110Combinednew
WHERE
raw_alc_R110Combinednew.OMC_ID = raw_alc_rmfscombinednew.OMC_ID
AND raw_alc_R110Combinednew.CELL_CI = raw_alc_rmfscombinednew.CI
AND RAW_ALC_R110COMBINEDNEW.START_TIME = RAW_ALC_RMFSCOMBINEDNEW.START_TIME
Results of Last execution:
Elapsed Time (sec) 4,476.56
CPU Time (sec) 4,471.88
Wait Time (sec) 4.68
Executions that Fetched all Rows (%) 100.00
Average Persistent Mem (KB) 32.43
Average Runtime Mem (KB) 31.59
--------------------------------------------------------------

Answer 1

Your update requires a full execution of the subquery for each row in the destination table (raw_alc_rmfscombinednew). Try rewriting as a MERGE. Tons faster.
MERGE INTO raw_alc_rmfscombinednew  D
      USING ( SELECT distinct
                     BSC,
                     CELL_NAME
                     START_TIME
                FROM raw_alc_R110Combinednew )  S
         ON (    S.OMC_ID     = D.OMC_ID
             AND S.CELL_CI    = D.CI
             AND S.START_TIME = D.START_TIME )
    WHEN MATCHED THEN
      UPDATE
         SET D.BSC        = S.BSC,
             D.CELL_NAME  = S.CELL_NAME