Understanding the Performance Comparison Between UNION and Non-UNION Queries

In this article, we will delve into the world of conditional aggregation queries in Oracle and explore the performance comparison between UNION and non-UNION queries. Specifically, we’ll examine a common use case where a query needs to aggregate data based on conditions applied to different columns.

Introduction to Conditional Aggregation

Conditional aggregation is a powerful feature in Oracle that allows you to perform aggregations based on specific conditions applied to individual rows. In this scenario, our goal is to calculate two separate sums for two different categories of customers based on their id values.

The Problem at Hand

The original query uses the UNION operator to combine two subqueries. One subquery calculates the sum for a particular category, and another subquery calculates the sum for an entirely different category. This approach results in slower performance compared to other alternatives that don’t require the use of UNION.

Without UNION: A More Efficient Approach

Let’s first consider the approach without using UNION. In this version, we simply combine the two conditions into a single CASE expression within the SELECT statement. The resulting query is much simpler and more efficient than its UNION-based counterpart.

SELECT 
    SUM(CASE WHEN gc.id = 11 THEN CB.CURRENT_TOTAL ELSE 0 END) AS "GOVT (Defence)",
    SUM(CASE WHEN gc.id != 11 THEN CB.CURRENT_TOTAL ELSE 0 END) AS "GOVT (Except Defence)"
FROM cust_domestic c
INNER JOIN GOVERNMENT_CUSTOMERS gc ON c.code = gc.cust_code
INNER JOIN customer_bill cb ON cb.cust_code = c.code;

The Benefits of This Approach

This revised query has several advantages over the original UNION-based approach:

1. Simplified Query Structure: By combining the two conditions into a single CASE expression, we’ve simplified the overall query structure.
2. Reduced Number of Subqueries: This approach eliminates the need for separate subqueries, which can improve performance by reducing the number of database operations.
3. Faster Execution Time: As the revised query is much simpler than its UNION-based counterpart, it tends to execute faster due to reduced overhead.

The Impact on Performance

To understand how this simplified approach affects overall performance, let’s examine some key aspects:

1. Reduced Overhead: By avoiding subqueries and using a single CASE expression, we’ve minimized the overhead associated with executing these queries.
2. Improved Indexing Efficiency: Since our revised query is more straightforward, it may take advantage of existing indexes on the columns involved, leading to faster execution times.
3. Increased Database Optimizer Effectiveness: Oracle’s database optimizer has a better chance of identifying optimal execution plans for this simplified query due to its reduced complexity.

The UNION Approach: A Simpler Alternative?

The original query using UNION is often a viable alternative when dealing with multiple conditions or subqueries. However, in the context described here, where we have two separate sums based on different conditions, the non-UNION approach provides an optimal solution.

Why Is This So?

While it might seem counterintuitive at first, the original query using UNION actually performs poorly due to:

1. Increased Overhead: Subqueries can lead to increased overhead during execution, including the need for extra database operations and memory allocation.
2. Reduced Indexing Efficiency: By breaking down a single query into multiple subqueries, we reduce the potential benefits of indexing on individual columns.

Conclusion

In conclusion, the revised query without UNION offers improved performance due to reduced overhead, increased indexing efficiency, and better optimizer effectiveness. This is particularly important when dealing with large datasets or complex queries where every millisecond counts.

While UNION can be a powerful tool in certain situations, this particular scenario clearly favors the simplified non-UNION approach. By taking advantage of Oracle’s conditional aggregation features and optimizing our query structure accordingly, we can unlock significant performance gains in our database applications.

Additional Considerations

There are other factors to consider when evaluating query performance:

• Query Optimization: Use tools like EXPLAIN PLAN or the Oracle Tuning Toolkit to analyze and optimize your queries.
• Indexing Strategies: Leverage indexes strategically to improve query execution times, particularly for frequently executed operations.
• Resource Allocation: Ensure adequate resources are allocated to the database server to prevent resource contention or performance degradation.

By keeping these factors in mind and applying best practices like those outlined above, you can create more efficient queries that meet your specific requirements while minimizing potential bottlenecks.

Last modified on 2023-06-04