Choosing Between CTEs and Temp Tables: Performance Impact

Introduction

When working with complex SQL queries, developers often face a choice between using Common Table Expressions (CTEs) and temporary tables. Both serve similar purposes—breaking down complex queries into manageable parts—but they have different performance characteristics and use cases. Understanding when to use each can significantly impact your query performance and maintainability.

What Are CTEs?

Common Table Expressions (CTEs) are named temporary result sets that exist only within the execution scope of a single SQL statement. Introduced with the WITH clause, CTEs help improve query readability without necessarily materializing the intermediate results.

WITH SalesCTE AS (
    SELECT product_id, SUM(quantity) as total_sales
    FROM orders
    GROUP BY product_id
)
SELECT p.product_name, s.total_sales
FROM products p
JOIN SalesCTE s ON p.product_id = s.product_id;

What Are Temporary Tables?

Temporary tables are physical tables that exist temporarily in the database. They are created in tempdb and can be indexed, have statistics, and persist for the duration of a session or transaction.

CREATE TABLE #TempSales (
    product_id INT,
    total_sales INT
);

INSERT INTO #TempSales
SELECT product_id, SUM(quantity)
FROM orders
GROUP BY product_id;

SELECT p.product_name, t.total_sales
FROM products p
JOIN #TempSales t ON p.product_id = t.product_id;

Performance Comparison

CTE Advantages:

1. Readability: CTEs make complex queries more readable by breaking them into logical components

2. No physical storage: CTEs don't create physical tables, reducing tempdb overhead

3. Recursion: CTEs support recursive queries (hierarchical data)

4. Query optimization: The optimizer can sometimes fold CTEs into the main query plan

Temporary Table Advantages:

1. Reusability: Can be referenced multiple times in a session

2. Indexing: Can have indexes created for performance

3. Statistics: SQL Server maintains statistics on temp tables

4. Reduced complexity: Break complex operations into simpler steps

5. Explicit materialization: Data is physically stored and can be reused

When to Use CTEs

1. For queries that will only be referenced once

2. When you need recursive capabilities

3. For simpler queries where readability is the primary concern

4. When working with query fragments that don't benefit from materialization

5. In environments where tempdb contention is a concern

When to Use Temporary Tables

1. When intermediate results need to be referenced multiple times

2. For complex queries that benefit from statistics and indexing

3. When working with large datasets where materialization improves performance

4. When you need to break a complex operation into discrete steps

5. In stored procedures where intermediate results are reused

Performance Considerations

1. CTEs are not always inlined: Modern SQL Server versions often materialize CTEs, especially complex ones

2. Temp tables have overhead: Creating and dropping temp tables adds overhead

3. Cardinality matters: For small datasets, CTEs often perform better; for large datasets, temp tables may win

4. Multiple references: Queries that reference a CTE multiple times may execute the CTE logic multiple times

5. Memory pressure: Temp tables can increase tempdb usage

Real-World Example

Consider a reporting query that:

1. Filters a large sales table

2. Joins with product information

3. Aggregates results

4. Joins the aggregates back to dimension tables

CTE approach might look cleaner but could result in the filtered sales data being processed multiple times.

Temp table approach would materialize the filtered sales data once, potentially with appropriate indexes, leading to better performance for large datasets.

Best Practices

1. Test both approaches: Always compare execution plans for your specific scenario

2. Consider query complexity: Simple queries favor CTEs; complex ones often benefit from temp tables

3. Monitor tempdb usage: Excessive temp table usage can lead to contention

4. Use table variables for small datasets: In some cases, they may be better than both options

5. Consider your SQL Server version: Newer versions have improved CTE optimization

Conclusion

There's no one-size-fits-all answer to whether CTEs or temporary tables are better. CTEs offer cleaner syntax and are often sufficient for simpler queries, while temporary tables provide more control and better performance for complex operations on large datasets. The key is to understand both tools and choose the right one based on your specific requirements, testing performance when in doubt.

Remember that query optimization is often about tradeoffs—between readability and performance, between memory usage and CPU cycles, and between development time and execution time. Having both CTEs and temporary tables in your SQL toolkit allows you to make the right choice for each situation.