Query Optimization with ROW_NUMBER

Introduction

As the amount of data in our databases continues to grow, the importance of optimizing queries becomes increasingly crucial. One technique that can significantly impact performance is using the ROW_NUMBER() function. In this article, we’ll explore how ROW_NUMBER() affects query optimization and provide strategies for improving performance.

Understanding ROW_NUMBER()

ROW_NUMBER() is a window function used to assign a unique number to each row within a partition of a result set. The syntax for ROW_NUMBER() in Oracle is:

SELECT 
    column1,
    column2,
    ROW_NUMBER() OVER (PARTITION BY column_group ORDER BY column_order) AS row_num
FROM table_name;

The first step in optimizing queries with ROW_NUMBER() is understanding how it works. When you use ROW_NUMBER(), the database needs to access all rows in the partition and window sort them according to the specified columns.

Analyzing the given query

Let’s analyze the given query:

SELECT 
    PE1.PRODUCT_EQUIPMENT_KEY, -- primary key
    PE1.Customer_Ban, 
    PE1.Subscriber_No, 
    PE1.Prod_Equip_Cd, 
    PE1.Prod_Equip_Txt, 
    PE1.Prod_Equip_Category_Txt--,
    -- PE2.ep_rnk ------------------ UNCOMMENT THIS LINE
FROM 
    INT_ADM.Product_Equipment_Dim PE1
    INNER JOIN 
    ( 
        SELECT 
            PRODUCT_EQUIPMENT_KEY,
            ROW_NUMBER() OVER (PARTITION BY Customer_Ban, Subscriber_No ORDER BY Start_Dt ASC) AS ep_rnk
            FROM INT_ADM.Product_Equipment_Dim PE2
    ) PE2
    ON PE2.PRODUCT_EQUIPMENT_KEY = PE1.PRODUCT_EQUIPMENT_KEY
WHERE 
    Line_Of_Business_Cd = 'M' 
    AND /*v_Date_Start*/ TO_DATE( '2022/01/12', 'yyyy/mm/dd' ) BETWEEN Start_Dt AND End_Dt 
    AND Current_Ind = 'Y' 

The query consists of two parts: the main query and a subquery. The subquery uses ROW_NUMBER() to assign a unique number to each row within a partition based on the Customer_Ban, Subscriber_No, and Start_Dt columns.

Effect of uncommenting ROW_NUMBER()

When the ROW_NUMBER() line is commented out, the query completes in under a second. However, when it’s uncommented, the query takes up to 5 minutes to complete. This drastic change in performance suggests that using ROW_NUMBER() significantly impacts the query’s execution time.

Understanding why ROW_NUMBER() slows down the query

The reason ROW_NUMBER() slows down the query is due to its inherent nature of needing to access all rows within a partition and window sort them according to the specified columns. This operation can be resource-intensive, especially when dealing with large datasets.

When you use ROW_NUMBER(), the database needs to:

1. Access all rows in the partition
2. Window sort the rows based on the specified columns
3. Join the sorted rows with the main query

These operations are necessary for obtaining the correct ranking of each row, but they can significantly impact performance.

Improving performance

To improve performance when using ROW_NUMBER(), consider the following strategies:

1. Filter the subquery

If the predicate in the WHERE clause filters the table very restrictive, make a similar filter in the subquery.

SELECT 
    PRODUCT_EQUIPMENT_KEY,
    ROW_NUMBER() OVER (PARTITION BY Customer_Ban, Subscriber_No ORDER BY Start_Dt ASC) AS ep_rnk
FROM INT_ADM.Product_Equipment_Dim
WHERE ... same predicate as in the main query ...

2. Pre-calculate rank

If the predicate returns all or most of the PRODUCT_EQUIPMENT_KEY values, consider pre-calculating the rank by creating a materialized view.

Creating a materialized view

Create a materialized view that includes the ranking:

CREATE MATERIALIZED VIEW mv_product_equipment_rank AS
SELECT 
    PE1.PRODUCT_EQUIPMENT_KEY, -- primary key
    PE1.Customer_Ban, 
    PE1.Subscriber_No, 
    PE1.Prod_Equip_Cd, 
    PE1.Prod_Equip_Txt, 
    PE1.Prod_Equip_Category_Txt--,
    ROW_NUMBER() OVER (PARTITION BY Customer_Ban, Subscriber_No ORDER BY Start_Dt ASC) AS ep_rnk
FROM INT_ADM.Product_Equipment_Dim PE1;

Then, create a simple query from the materialized view without joining it with the main query.

3. Materialize view creation

If you have access to the database administrator, consider creating a materialized view that includes the ranking.

CREATE MATERIALIZED VIEW mv_product_equipment_rank AS
SELECT 
    PE1.PRODUCT_EQUIPMENT_KEY, -- primary key
    PE1.Customer_Ban, 
    PE1.Subscriber_No, 
    PE1.Prod_Equip_Cd, 
    PE1.Prod_Equip_Txt, 
    PE1.Prod_Equip_Category_Txt--,
    ROW_NUMBER() OVER (PARTITION BY Customer_Ban, Subscriber_No ORDER BY Start_Dt ASC) AS ep_rnk
FROM INT_ADM.Product_Equipment_Dim PE1;

This materialized view can be queried directly without the need for joining it with another query.

Conclusion

In conclusion, using ROW_NUMBER() can significantly impact performance due to its inherent nature of needing to access all rows within a partition and window sort them according to the specified columns. By understanding how ROW_NUMBER() works and applying strategies such as filtering the subquery or creating a materialized view, you can improve the performance of your queries.

Best practices for using ROW_NUMBER()

To avoid performance issues when using ROW_NUMBER():

• Filter the subquery whenever possible
• Pre-calculate rank by creating a materialized view if the predicate returns all or most of the required values
• Materialize views can be used to simplify complex queries and improve performance

Last modified on 2024-02-23