optimize the loop and increase performance

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VB

Oracle

I have dataset with 1 crore data and i am looping through it and using values of dataset for updating the base table of count 2.5 crore.

The fields which is used in where clause is primary key

But still i am not getting performance only 1 lac records getting updated in 1 hr.

How do i optimize and speed up time to execute query in oracle. I am using command object, Coneection object in vb.net

Please help.

following function executed 1 crore time to update query.

public cmd_obj as new oledb.oledbcommand
Strquery="update Table1 set field1='Value from dataset' where field2='value from dataset'" ' where field2 is primary key in base table
Public Function Executenonquery(ByVal Strquery As String) As Int32
Dim intUpdated As Int32 = 0
Try
If VerifyOracleLogin() = True Then ' check oracle connection
cmd_obj.Connection = dbconn
cmd_obj.CommandText = Strquery 'strquery has update query
intUpdated = cmd_obj.ExecuteNonQuery()
return intUpdated
End If   
Catch ex As Exception
WriteToErrorLog("Query Execution Error : " & Strquery, ex.Message)
Return -1
End Try
End Function