Optimizing sybase SQL Query

Is there a way to optimize the following query:

UPDATE myTable
SET Calculation =
  (SELECT MAX(Calculation)
   开发者_JS百科FROM myTable T
   WHERE T.Id = myTable.Id
     AND T.Flag='N')
WHERE Calculation='NA'
  AND Flag='Y'

where myTable has approx. 4 million rows? Actually the first not NULL will do the job (SYBASE ASE 15.0.2).

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Check Query plan, it must be using deferred update which is taking longer time to update. Query suggested by Michael should perform better.

rememeber below points which require deferred update

• Updates that use self-joins

• Updates to columns used for self-referential integrity

• Updates to a table referenced in a correlated subquery

Thanks..