[PR] Fix exists() cross-join caused by duplicate CriteriaQuery root [grails-core]

["jamesfredley (via GitHub)" <gi...@apache.org>]

jamesfredley opened a new pull request, #15419:
URL: https://github.com/apache/grails-core/pull/15419

   ## Summary
   
   Fixes #14334
   
   `GormEntity.exists()` was performing a cartesian product (cross-join) against the entire table due to `criteriaQuery.from()` being called twice in `AbstractHibernateGormStaticApi.exists()`.
   
   Reproducer: https://github.com/scottlollman/grails-data-mapping-issue-2071
   
   ## Root Cause
   
   `AbstractHibernateGormStaticApi.exists()` created two query roots:
   
   ```groovy
   Root queryRoot = criteriaQuery.from(persistentEntity.javaClass)          // root 1
   // ...
   criteriaQuery.select(criteriaBuilder.count(criteriaQuery.from(...)))     // root 2 (BUG)
   ```
   
   Each call to `criteriaQuery.from()` adds a new root to the query. Two roots on the same table produce a cross-join:
   
   ```sql
   -- Before (cross-join scans entire table)
   select count(generatedAlias0)
   from SomeDomain as generatedAlias1, SomeDomain as generatedAlias0
   where generatedAlias1.id=1L
   
   -- After (single root, simple indexed lookup)
   select count(generatedAlias0)
   from SomeDomain as generatedAlias0
   where generatedAlias0.id=1L
   ```
   
   While the boolean result was technically correct (non-zero = true), the query performed a full table scan for every `exists()` call.
   
   ## Fix
   
   One-line change - reuse the existing `queryRoot` variable instead of calling `criteriaQuery.from()` a second time:
   
   ```groovy
   criteriaQuery.select(criteriaBuilder.count(queryRoot))
   ```
   
   ## Tests
   
   `ExistsCrossJoinSpec` - 4 integration tests using a real `HibernateDatastore` with H2:
   - `exists` returns true for existing entity
   - `exists` returns false for non-existent id
   - `exists` does not produce a cross-join (SQL captured via Hibernate `StatementInspector`, verified no cross-join or comma-join pattern)
   - `exists` with multiple rows returns correct result
   
   Existing `ReadOperationSpec` tests continue to pass.


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Re: [PR] Fix exists() cross-join caused by duplicate CriteriaQuery root [grails-core]

["jdaugherty (via GitHub)" <gi...@apache.org>]

jdaugherty merged PR #15419:
URL: https://github.com/apache/grails-core/pull/15419


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