How Can I Efficiently Implement Union Queries Without Hibernate's Direct Support?

Alternatives to Hibernate Union Queries

Hibernate currently lacks support for union queries. As a result, implementing them requires exploring alternative approaches.

One option is to leverage MySQL's subselect feature to emulate union queries. This involves using "id in (select...)" or "id in (select...)" expressions within the where clause. However, this approach can lead to performance issues in certain scenarios.

Another alternative is to utilize plain JDBC. While this method enables direct database interaction, it sacrifices the benefits of Hibernate's example/criteria queries and mapping validation.

For improved performance, it may be beneficial to perform two simple queries and manually merge the results. This approach avoids the complexities of subselect queries and provides a more efficient solution.

To illustrate the potential performance implications, consider the following MySQL query:

select p.* from PERSON p 
where p.id in (select p1.id from PERSON p1 where p1.name = "Joe") 
or p.id in (select p2.id from PERSON p2 
join CHILDREN c on p2.id = c.parent where c.name="Joe")

The resulting EXPLAIN output reveals that the query does not use any index and considers over 200,000 rows, significantly impacting performance. In contrast, executing two subqueries independently typically takes only milliseconds.

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