Understanding UNION and UNION ALL in Oracle SQL: Why UNION ALL is Faster

In the realm of database management, particularly within the Oracle SQL environment, it's essential to understand the performance implications of using UNION versus UNION ALL. Both operators are designed to combine the results of multiple SELECT statements, but they differ significantly in how they handle the results - one of which results in a faster performance due to its simpler operation. This article explores the fundamental differences between these two union operators and why UNION ALL often outperforms UNION in terms of speed.

The Basics of Union Operators in Oracle SQL

UNION and UNION ALL are used to combine the results from different SELECT statements into one result set. The primary differences between these two operators lie in their functionality and performance implications, which are detailed below.

What Does UNION Do?

UNION is a simple yet powerful operator designed to combine the results from multiple SELECT statements into a single set. However, UNION is also equipped with the ability to automatically eliminate duplicate rows from the final result set. This automatic duplication elimination means that UNIONS need to perform additional operations, which in some cases can significantly impact performance.

What Does UNION ALL Do?

Unlike its counterpart, UNION ALL does not attempt to remove duplicate rows from the result set. Instead, it combines the results from all the input SELECT statements without any duplication, making it a much more straightforward operation. This absence of extra processing steps is what contributes to the superior performance of UNION ALL in comparison to UNION.

Hands-on Example: Difference Between UNION and UNION ALL

To illustrate the difference between UNION and UNION ALL, let's consider a simple example. Assume we have two tables, Table1 and Table2, with the following data:

Table1 contains the records {A B C} Table2 contains the records {A B}

The results of the respective queries using UNION and UNION ALL would be as follows:

UNION: {A B C} UNION ALL: {A B C A B}

As evident, UNION automatically removed the duplicate 'A B' entry, while UNION ALL preserved it, adding an extra step for the database to process.

Why UNION ALL is Faster Than UNION

The primary reason for the performance difference between UNION and UNION ALL lies in the operations they undertake. UNION has to perform additional steps to identify and remove duplicate rows, which adds computational overhead to the query execution process. This extra processing step can be time-consuming, especially with large data sets or complex queries.

Conceptually, when a database is executing a UNION query, it follows these steps:

Executes all the constituent SELECT statements in the UNION operation. Merges the results. Removes duplicate rows.

In contrast, a UNION ALL query simplifies the process to the bare essentials:

Executes all the constituent SELECT statements in the UNION ALL operation. Merges the results without any additional processing.

These differences in processing steps directly translate to reduced computational load and faster execution times when using UNION ALL, particularly with larger data sets or in scenarios where duplicate removal is not required or desired.

When to Use UNION and UNION ALL

The choice between UNION and UNION ALL in Oracle SQL hinges on the specific requirements of your query. You should opt for UNION when you need to ensure the uniqueness of the results and don't require duplicates. On the other hand, if your application or business logic mandates the inclusion of all results or if you're working in a context where duplicate removal is unnecessary, UNION ALL is the more efficient choice.

For example, consider a scenario where you are attempting to display a comprehensive set of footprints from different sources, or you are performing an analysis that benefits from a more inclusive result set. In such cases, the absence of duplicate removal should be leveraged to your advantage.

Conclusion

In conclusion, the choice between UNION and UNION ALL in Oracle SQL should be made based on the specific needs of your application or query. UNION provides a robust mechanism for ensuring the uniqueness of the result set, but it comes at the cost of additional computational overhead. UNION ALL, on the other hand, offers a faster execution time and simpler operation which is advantageous in many real-world scenarios. Understanding these subtle differences can help you write more efficient and effective SQL queries.

Related Keywords

Oracle SQL Union Union ALL Duplicates Performance