SQL Performance: When Should I Use UNION Instead of OR?

SQL Performance: UNION vs. OR Comparison

In database optimization, it is usually recommended to replace the OR statement with a UNION statement. However, the potential performance impact of this approach is a matter of concern.

Explanation of OR and UNION

This article recommends using UNION to replace the OR statement when querying tables. For example:

<code class="language-sql">select username from users where company = 'bbc' or company = 'itv';</code>

can be rewritten as:

<code class="language-sql">select username from users where company = 'bbc' union
select username from users where company = 'itv';</code>

Performance impact

Judging from the EXPLAIN output, UNION seems to double the work. However, this is not the case in all cases.

For simple OR conditions involving only a single column, MySQL can efficiently use indexes to retrieve data. In this case, UNION does not bring significant performance benefits.

The advantage of UNION is when the OR condition involves different columns. Consider the following query:

<code class="language-sql">select username from users where company = 'bbc' or city = 'London';</code>

In this case, MySQL cannot use two indexes (one for company and one for city) at the same time. UNION provides a way to split the search into two subqueries, each of which can take advantage of the corresponding index.

Conclusion

Although UNION can effectively optimize complex OR conditions, it does not always definitely double the workload. The best approach depends on the specific query and the distribution of data in the table. It is recommended to evaluate OR and UNION statements in MySQL Query Analyzer to determine the best solution.

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