Incorrect ordering in CTE query

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Summary

The issue at hand is related to the incorrect ordering of rows in a query using a Common Table Expression (CTE) with ROW_NUMBER() function. The query returns the correct ordering when run standalone, but the ordering is incorrect when wrapped in a CTE.

Root Cause

The root cause of this issue is due to the non-deterministic nature of the ROW_NUMBER() function. This function assigns a unique number to each row within a partition of a result set, but the order of the rows is not guaranteed to be the same every time the query is run. The reasons for this include:

  • The ORDER BY clause in the ROW_NUMBER() function is not sufficient to guarantee a specific order
  • The CTE may be optimized differently by the query optimizer, leading to a different order of rows

Why This Happens in Real Systems

This issue can occur in real systems due to various reasons, including:

  • Lack of a unique identifier in the data, making it difficult to guarantee a specific order
  • Query optimization techniques used by the database engine, which can change the order of rows
  • Parallel processing of queries, which can also affect the order of rows

Real-World Impact

The impact of this issue can be significant, including:

  • Incorrect results being returned to the user
  • Inconsistent data being used for reporting or analysis
  • Difficulty in troubleshooting issues due to the non-deterministic nature of the query

Example or Code

-- Create a sample table
CREATE TABLE dbo.vw_xyz (
    management_id INT,
    management_full_name VARCHAR(100),
    management_email VARCHAR(100),
    compl_officer_id INT,
    compl_officer_full_name VARCHAR(100),
    compl_officer_email VARCHAR(100)
);

-- Insert sample data
INSERT INTO dbo.vw_xyz (management_id, management_full_name, management_email, compl_officer_id, compl_officer_full_name, compl_officer_email)
VALUES
(104334, 'Management 1', 'management1@example.com', 297898, 'Compliance Officer 1', 'compliance1@example.com'),
(104334, 'Management 1', 'management1@example.com', 297898, 'Compliance Officer 1', 'compliance1@example.com'),
(104334, 'Management 1', 'management1@example.com', 297898, 'Compliance Officer 1', 'compliance1@example.com');

-- Run the query with ROW_NUMBER()
SELECT management_id, management_full_name, management_email, compl_officer_id, compl_officer_full_name, compl_officer_email, ROW_NUMBER() OVER(PARTITION BY management_id ORDER BY compl_officer_id) AS row_num
FROM dbo.vw_xyz
WHERE management_id = 104334 AND compl_officer_id = 297898;

-- Run the query with CTE
WITH cte1 AS (
    SELECT management_id, management_full_name, management_email, compl_officer_id, compl_officer_full_name, compl_officer_email, ROW_NUMBER() OVER(PARTITION BY management_id ORDER BY compl_officer_id) AS row_num
    FROM dbo.vw_xyz
    WHERE management_id = 104334 AND compl_officer_id = 297898
)
SELECT * FROM cte1;

How Senior Engineers Fix It

To fix this issue, senior engineers can use various techniques, including:

  • Adding a unique identifier to the data to guarantee a specific order
  • Using a deterministic sorting method, such as sorting by a unique identifier
  • Avoiding the use of ROW_NUMBER() and instead using a different method to assign a unique number to each row

Why Juniors Miss It

Junior engineers may miss this issue due to:

  • Lack of understanding of the non-deterministic nature of the ROW_NUMBER() function
  • Insufficient testing of the query to ensure that the results are consistent
  • Failure to consider the impact of query optimization and parallel processing on the order of rows

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