Best way to update 40 million rows in batch

Question

Basically I need to run this on a table with 40 million rows, updating every row at once will crash, so I want to batch the query so that if it crash, it can re-run the quer

Answer 1

M.Ali's suggestion will work, but you will end up with degrading performance as you work through the 40M records. I would suggest a better filter to find the records to update in each pass. This would assume you have a primary key (or other index) on your identity column:

DECLARE @Rowcount INT = 1
    ,   @BatchSize INT = 100000
    ,   @StartingRecord BIGINT = 1;

WHILE (@Rowcount > 0)   
BEGIN
    UPDATE [table]
        SET [New_ID] = [Old_ID]
    WHERE [table_ID] BETWEEN @StartingRecord AND @StartingRecord + @BatchSize - 1;

    SET @Rowcount = @@ROWCOUNT;

    CHECKPOINT;

    SELECT @StartingRecord += @BatchSize
END

This approach will allow each iteration to be as fast as the first. And if you don't have a valid index you need to fix that first.