Add support for subquery modifiers (LIMIT/OFFSET) in Hibernate

[DongGunYoon]

Description

This PR addresses a long-standing limitation in QueryDSL where LIMIT and OFFSET modifiers were ignored when used within subqueries. This change enables full subquery modifier support specifically for Hibernate users while maintaining backward compatibility and JPQL standard compliance for other JPA providers.

Related Issue: This resolves the subquery modifier limitation reported in querydsl/querydsl#3224

Problem Statement

Previously, when users applied .limit() or .offset() to subqueries, modifiers were silently ignored during query serialization. This was particularly frustrating for Hibernate users since Hibernate 6.0+ natively supports LIMIT/OFFSET in subqueries, but QueryDSL wasn't utilizing this capability.

Example of previous behavior:

// This would ignore the .limit(1) in the subquery
query.select(cat)
    .from(cat)
    .where(cat.id.in(
        JPAExpressions.select(mate.id)
            .from(mate)
            .where(mate.weight.gt(5))
            .limit(1)  // ← This was ignored!
    ))
    .fetch();

Solution

This implementation introduces a template-based approach that allows JPA provider-specific handling of subquery modifiers:

🔧 Core Changes

1. Template Support Detection

• Added isSubQueryModifiersSupported() method to JPQLTemplates
• Returns false by default (standard JPQL compliance)
• Overridden in HQLTemplates to return true (Hibernate-specific support)

2. Conditional Serialization

• Enhanced JPQLSerializer.visit(SubQueryExpression) to conditionally serialize modifiers
• Only applies LIMIT/OFFSET when the template supports it
• Preserves existing behavior for all other JPA providers

3. Serialization Constants

• Added LIMIT and OFFSET constants for consistent formatting
• Follows existing QueryDSL serialization patterns

📋 Supported Scenarios

• ✅ Subqueries in WHERE clauses with LIMIT/OFFSET
• ✅ Scalar subqueries in SELECT clauses with modifiers
• ✅ Nested subqueries with independent modifiers
• ✅ Combined LIMIT and OFFSET usage

Compatibility

✅ Backward Compatibility

• Zero breaking changes to existing APIs
• All existing queries continue to work exactly as before
• No impact on users not using subquery modifiers

🎯 Provider Support

• Hibernate: ✅ Full subquery modifier support (new feature)
• EclipseLink: ⭕ No change (maintains current behavior)
• OpenJPA: ⭕ No change (maintains current behavior)
• DataNucleus: ⭕ No change (maintains current behavior)
• BatooJPA: ⭕ No change (maintains current behavior)

Technical Implementation Details

The implementation uses a template-based approach to maintain provider neutrality:

1. JPQLTemplates.isSubQueryModifiersSupported(): Base method returns false (JPQL standard)
2. HQLTemplates.isSubQueryModifiersSupported(): Override returns true (Hibernate extension)
3. JPQLSerializer: Conditionally serializes modifiers based on template support

This design ensures that:

• Standard JPQL compliance is maintained for most providers
• Hibernate-specific features are enabled only when appropriate
• Future providers can easily add support by overriding the template method

[codecov]

Codecov Report

❌ Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (9b9e4fc) to head (95dd7be).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...src/main/java/com/querydsl/jpa/JPQLSerializer.java	0.00%	10 Missing ⚠️
...a/src/main/java/com/querydsl/jpa/HQLTemplates.java	0.00%	1 Missing ⚠️
.../src/main/java/com/querydsl/jpa/JPQLTemplates.java	0.00%	1 Missing ⚠️

Additional details and impacted files

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  Coverage    0.00%   0.00%            
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  Files         812     837    +25     
  Lines       31138   31479   +341     
  Branches     3531    3544    +13     
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- Misses      31138   31479   +341     

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[velo]

Looking good, but can you please add a test to the HibernateBase, to test it for reals too.

[DongGunYoon]

Looking good, but can you please add a test to the HibernateBase, to test it for reals too.

Thanks for the feedback! I've added comprehensive integration tests to HibernateBase that verify the actual functionality of subquery modifiers with Hibernate execution.