SQL Performance Optimization: When is UNION Better Than OR?

SQL performance optimization: comparison of UNION and OR

In database optimization, understanding the difference between UNION and OR is crucial to query performance.

This article explores ways to replace OR statements with UNION to improve performance. However, upon closer analysis, the examples provided are misleading.

In MySQL, the following query:

select username from users where company = 'bbc' or company = 'itv';

is equivalent to:

select username from users where company IN ('bbc', 'itv');

Since MySQL can effectively use the index on the company column in both queries, the UNION optimization is redundant in this case.

When the OR condition involves different columns, the advantages of the UNION method are even more obvious. Consider the following:

select username from users where company = 'bbc' or city = 'London';

In this case, MySQL faces a difficulty that only one index per table can be used in a query. UNION solves this problem by splitting the query into two subqueries:

select username from users where company = 'bbc'
union
select username from users where city = 'London';

Now, each subquery can utilize the corresponding index and the results are merged by UNION.

However, UNION does incur a slight overhead of sorting the result set to eliminate duplicates. This may slow down queries. Still, in most cases, the efficiency gain from using the right index outweighs the cost of sorting.

Ultimately, the best choice between UNION and OR depends on the specific data and query structure. Testing both approaches in MySQL Query Analyzer can provide empirical evidence to determine the best solution.

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