Computed Columns and Persisted Columns

Computed Columns and Persisted Columns in SQL Server: A Detailed Guide

Introduction

SQL Server provides various ways to store and manipulate data efficiently. One of the essential features of SQL Server is the concept of computed columns and persisted columns, which can be used to store values dynamically based on other columns in a table. These features allow for increased flexibility, performance optimization, and maintainability in database design. Understanding when and how to use these columns is critical for both developers and database administrators, especially in large-scale, performance-critical applications.

In this guide, we will explore computed columns and persisted columns in SQL Server, explain how they work, their differences, and best practices. We will cover:

What computed and persisted columns are.
The difference between computed and persisted columns.
How to create and use them.
Practical examples and use cases.
Performance considerations.
Best practices.

1. What are Computed Columns?

A computed column is a virtual column in a SQL Server table whose value is calculated from an expression involving one or more other columns in the same table. This column does not physically store data but computes its value each time it is queried.

Key Characteristics of Computed Columns:

Expression-Based: The value of a computed column is derived from an expression that uses other columns from the same table.
Not Physically Stored: Unlike regular columns, computed columns do not store data but calculate the value on the fly when queried.
Dynamic: The value of a computed column is automatically updated whenever the values of the underlying columns change.
Read-Only: Computed columns are read-only; you cannot directly insert or update the value of a computed column.

Computed Column Syntax:

CREATE TABLE TableName (
    Column1 INT,
    Column2 INT,
    ComputedColumn AS (Column1 + Column2)
);

In this example, ComputedColumn will automatically store the sum of Column1 and Column2. The computed column value is evaluated dynamically whenever a query referencing ComputedColumn is executed.

Examples of Computed Columns:

Basic Computed Column: If you have a Products table with Price and TaxRate columns, you can create a computed column to calculate the tax amount for each product: CREATE TABLE Products ( ProductID INT, ProductName VARCHAR(100), Price DECIMAL(10, 2), TaxRate DECIMAL(5, 2), TaxAmount AS (Price * TaxRate / 100) ); Here, TaxAmount will be automatically computed as the product of Price and TaxRate every time it is queried.
Complex Computed Column: You can also perform more complex operations, such as string concatenation or conditional logic. For example: CREATE TABLE Employees ( EmployeeID INT, FirstName VARCHAR(50), LastName VARCHAR(50), FullName AS (FirstName + ' ' + LastName) ); In this example, FullName is a computed column that combines FirstName and LastName.

Computed Columns with Functions:

Computed columns can also involve SQL functions like ROUND, CONVERT, or other mathematical operations:

CREATE TABLE Orders (
    OrderID INT,
    Quantity INT,
    UnitPrice DECIMAL(10, 2),
    TotalAmount AS (ROUND(Quantity * UnitPrice, 2))
);

In this case, TotalAmount is a computed column that calculates the total amount based on the quantity and unit price, rounded to two decimal places.

2. What are Persisted Columns?

A persisted column is a computed column that is physically stored in the table. This means the value of a persisted column is calculated when the data is inserted or updated and stored as part of the row. This is different from a regular computed column, where the value is calculated dynamically each time it is accessed.

Key Characteristics of Persisted Columns:

Physically Stored: The value of a persisted column is stored in the table, just like a regular column.
Calculated Once: The value is calculated once when data is inserted or updated, and stored in the table.
Performance Benefits: Persisted columns can improve performance, especially for frequently queried expressions, because SQL Server does not need to recompute the value each time it is queried.
Updateable: Since the value is physically stored, persisted columns can be indexed, and their values can be updated through INSERT or UPDATE operations.

Persisted Column Syntax:

CREATE TABLE TableName (
    Column1 INT,
    Column2 INT,
    PersistedColumn AS (Column1 + Column2) PERSISTED
);

The PERSISTED keyword indicates that the computed column is persisted (physically stored).

Examples of Persisted Columns:

Persisted Computed Column: CREATE TABLE Products ( ProductID INT, ProductName VARCHAR(100), Price DECIMAL(10, 2), TaxRate DECIMAL(5, 2), TaxAmount AS (Price * TaxRate / 100) PERSISTED ); In this example, TaxAmount is calculated and stored as part of the row in the Products table.
Persisted Computed Column with String Operations: CREATE TABLE Employees ( EmployeeID INT, FirstName VARCHAR(50), LastName VARCHAR(50), FullName AS (FirstName + ' ' + LastName) PERSISTED ); The FullName is a persisted computed column. The full name is calculated once when the row is inserted or updated and stored in the table.

3. Computed Columns vs Persisted Columns: Key Differences

Aspect	Computed Column	Persisted Column
Storage	Not physically stored in the table.	Physically stored in the table.
Calculation	Calculated dynamically each time the column is queried.	Calculated once when the data is inserted or updated.
Performance	May cause slower performance, especially for complex expressions, because the value is recalculated each time.	Faster queries since the value is precomputed and stored.
Indexing	Cannot be indexed directly.	Can be indexed, improving performance for queries that use the column.
Updateability	Cannot be directly updated.	Can be updated if the underlying columns are updated.
Use Cases	Useful for dynamic, real-time computations that do not need to be stored.	Useful for computations that need to be queried frequently or require indexing.

4. When to Use Computed Columns vs Persisted Columns

When to Use Computed Columns:

Dynamic Calculations: Use computed columns when you need to calculate values dynamically and don’t require them to be stored in the table. For instance, if the computation is relatively simple and doesn’t involve expensive operations, computed columns can be helpful.
Lightweight Calculations: If the expression in the computed column is lightweight and querying speed is not a concern, a computed column is often more flexible.
Avoid Redundancy: When you want to avoid storing redundant data. Computed columns can save space because they don’t physically store data, only the expression.
Non-Frequently Queried: If the computed column is not used frequently in queries or if the computed value doesn’t need to be indexed, computed columns are sufficient.

When to Use Persisted Columns:

Performance Optimization: If the computed value is queried frequently and performance is important, persisted columns can help by storing the precomputed value in the table.
Indexing: Persisted columns can be indexed, which can significantly speed up query performance when filtering or sorting by the computed value.
Frequent Updates: If the underlying columns are updated regularly, persisted columns can be recalculated and stored automatically, reducing the computation overhead during querying.
Large and Complex Calculations: When the computation is complex or involves joining large tables, persisted columns can be beneficial because the value is precomputed and stored, saving time on complex calculations during each query.

5. Performance Considerations with Computed and Persisted Columns

While both computed and persisted columns can improve the readability and organization of data in a database, there are performance considerations to take into account:

1. Computed Columns:

Query Performance: Since computed columns are not stored, the value is recalculated every time it is queried. For large datasets or complex expressions, this can lead to slower query performance.
No Indexing: Computed columns cannot be indexed, which can hinder performance if the column is used in WHERE, ORDER BY, or JOIN clauses.
Complexity of Expression: The more complex the expression, the more time it takes to compute each time. This can lead to a higher CPU load during query execution.

2. Persisted Columns:

Faster Queries: Since the value is precomputed and stored, queries involving persisted columns tend to run faster, especially for complex expressions or frequently used computations.
Indexing: Persisted columns can be indexed, which can improve query performance when the column is used in filtering or sorting.
Storage Overhead: Persisted columns require additional storage in the database, which can be a concern if there are many rows and the table contains numerous persisted computed columns.
Updates: If the underlying columns change frequently, the persisted columns must be recalculated and updated