使用分区依据选择每个组的最大值

Question

I have the following code which is taking a looong time to get executed. What I need to do is select the column having row number equals 1 after partitioning it by three columns (col_1, col_2, col_3) [which are also the key columns] and ordering by some columns as mentioned below. The number of records in the table is around 90 million. Am I following the best approach or is there any other better one?

  with cte as (SELECT
     b.*
    ,ROW_NUMBER() OVER ( PARTITION BY col_1,col_2,col_3
                         ORDER BY new_col DESC, new_col_2 DESC, new_col_3 DESC  ) AS ROW_NUMBER
  FROM (
    SELECT
       *
      ,CASE
         WHEN update_col = '        ' THEN new_update_col
         ELSE update_col
       END AS new_col_1
    FROM schema_name.table_name
    ) b
  )
 select top 10 * from cte WHERE ROW_NUMBER=1

Answer 1

Currently you are applying CASE on different columns which is impacting all rows in the database table. CASE (String Comparison) Is a costly method.

At the end, you are keeping only records with ROW NUMBER = 1. If I guess this filter keeping Half of your all records, this will increase the query execution time if you filter (Generate ROW NUMBER First and Keep Rows with RN=1) first and then apply CASE method on columns.