SQL Server Replication Issues with Computed Columns

Introduction:

In SQL Server, computed columns are a powerful feature that allows you to create virtual columns with values calculated on-the-fly based on other columns' data. While computed columns can simplify data retrieval and improve data consistency, they can also pose challenges when it comes to replication. This blog post explores the issues associated with computed columns in SQL Server replication and presents solutions to tackle them.

Understanding Computed Columns:

A computed column is essentially a virtual column within a table. It does not physically store its data in the table unless explicitly marked as "PERSISTED." Instead, a computed column derives its value based on an expression that can use data from other columns in the same table.

Challenges in Replication:

Replication in SQL Server involves copying data from one database (the publisher) to one or more destination databases (subscribers). However, when dealing with computed columns, replication can encounter obstacles. This is because replication attempts to insert or update data for the computed column, which is not allowed since the column's value is determined by the expression defined in it.

Table:

CREATE TABLE Employee (

    EmployeeID INT PRIMARY KEY,

    FirstName VARCHAR(50),

    LastName VARCHAR(50),

    Birthdate DATE,

    Age AS DATEDIFF(YEAR, Birthdate, GETDATE())  -- Computed column for age

);

Error in Replication:

One common error encountered when replicating computed columns is:

Error: The column "age" cannot be modified because it is either a computed column or is the result of a UNION operator.

This error message indicates that replication is trying to modify the "age" column, which is a computed column. Since computed columns are read-only and their values are determined by expressions, they cannot be modified directly during replication.

Solutions to Replication Issues with Computed Columns:To address these challenges, several options are available:

1. Exclude Computed Columns: Exclude computed columns from replication. This approach allows you to perform the computations at the replication destination database, ensuring that the computed values are correctly derived.
2. Remove Computed Expression: Convert the computed column into a regular column in the replication database by removing the computed expression. This way, the column becomes a regular, writable column that can be easily replicated.
3. Set "PersistantReadOnly" Property: Change the "kind" property of the computed column to "PersistantReadOnly" in the replication database. This adjustment helps replication understand that the column should be treated as a read-only, persistent column.

Conclusion:

Computed columns offer significant benefits in SQL Server, but they can introduce complications when dealing with replication. Understanding these issues and implementing the appropriate solution, such as excluding, modifying, or configuring the computed columns, is crucial to ensure seamless data replication in your SQL Server environment. By making informed choices, you can harness the power of computed columns while maintaining a robust replication process.