hibernate-demo/src/test/java/com/ankurm/hibernatedemo/cache/QueryCacheWithoutEntityCacheTest.java

package com.ankurm.hibernatedemo.cache;

import static org.assertj.core.api.Assertions.assertThat;

import java.util.List;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.boot.Metadata;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.query.Query;
import org.hibernate.stat.Statistics;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.Test;

/**
 * Checks the original article's claim -- "Query Cache without Entity Cache causes N+1 selects" --
 * by actually measuring it rather than repeating it. The claim does NOT hold: with {@link
 * UncachedProduct} carrying no {@code @Cacheable}/{@code @Cache} at all, a second, brand-new
 * session repeating the same {@code setCacheable(true)} query fires ZERO SQL statements, not five.
 *
 * <p>The query cache region does not store only the row ids. It stores the full hydrated tuple
 * state of each result row (id plus every mapped column) at the moment the query first ran.
 * Hibernate reconstitutes {@code UncachedProduct} instances directly from that stored tuple data
 * on a cache hit -- no re-query of the rows, and (confirmed below via {@code
 * getSecondLevelCacheHitCount()}) no dependency on the entity's own L2 region at all, because
 * {@link UncachedProduct} does not have one. This test does not rule out an N+1 appearing for a
 * query that returns associations Hibernate must still initialize per row, or for a partial-hit
 * scenario after individual cache entries are evicted -- only the specific, simple case the
 * article described (a flat entity, no associations, a repeated identical query) is measured and
 * corrected here.
 *
 * <p>Docs: docs/18-ehcache-l2-configuration.md
 */
class QueryCacheWithoutEntityCacheTest {

    private StandardServiceRegistry registry;
    private SessionFactory sessionFactory;

    private void boot() {
        registry = new StandardServiceRegistryBuilder()
                .applySetting("hibernate.connection.driver_class", "org.h2.Driver")
                .applySetting("hibernate.connection.url", "jdbc:h2:mem:querycachenoentity;DB_CLOSE_DELAY=-1")
                .applySetting("hibernate.connection.username", "sa")
                .applySetting("hibernate.connection.password", "")
                .applySetting("hibernate.hbm2ddl.auto", "create")
                .applySetting("hibernate.generate_statistics", "true")
                .applySetting("hibernate.cache.use_second_level_cache", "true")
                .applySetting("hibernate.cache.use_query_cache", "true")
                .applySetting("hibernate.cache.region.factory_class", "jcache")
                .applySetting("hibernate.javax.cache.provider", "org.ehcache.jsr107.EhcacheCachingProvider")
                .applySetting("hibernate.javax.cache.uri", "ehcache-chapter18.xml")
                .build();
        Metadata metadata = new MetadataSources(registry)
                .addAnnotatedClass(UncachedProduct.class)
                .buildMetadata();
        sessionFactory = metadata.buildSessionFactory();
    }

    @AfterEach
    void tearDown() {
        if (sessionFactory != null) {
            sessionFactory.close();
        }
        if (registry != null) {
            StandardServiceRegistryBuilder.destroy(registry);
        }
    }

    @Test
    void cachedQuery_overUncachedEntities_repeatRunIsWorseThanNoCachingAtAll() {
        boot();
        Statistics stats = sessionFactory.getStatistics();

        try (Session seed = sessionFactory.openSession()) {
            Transaction tx = seed.beginTransaction();
            for (int i = 1; i <= 5; i++) {
                seed.persist(new UncachedProduct("Widget " + i));
            }
            tx.commit();
        }

        String hql = "select p from UncachedProduct p order by p.id";

        stats.clear();
        List<UncachedProduct> first;
        try (Session s1 = sessionFactory.openSession()) {
            Query<UncachedProduct> q = s1.createQuery(hql, UncachedProduct.class);
            q.setCacheable(true);
            first = q.list();
        }
        long queriesForFirstRun = stats.getPrepareStatementCount();
        long queryCacheMissesAfterFirst = stats.getQueryCacheMissCount();

        stats.clear();
        List<UncachedProduct> second;
        try (Session s2 = sessionFactory.openSession()) {
            Query<UncachedProduct> q = s2.createQuery(hql, UncachedProduct.class);
            q.setCacheable(true);
            second = q.list();
        }
        long queriesForSecondRun = stats.getPrepareStatementCount();
        long queryCacheHitsAfterSecond = stats.getQueryCacheHitCount();
        long l2EntityHitsAfterSecond = stats.getSecondLevelCacheHitCount();

        System.out.println("RESULT[cache-query-without-entity-cache]: first run (cold, new session) SQL statements="
                + queriesForFirstRun + ", query-cache misses=" + queryCacheMissesAfterFirst
                + " | second run (new session, query-cache HIT) SQL statements=" + queriesForSecondRun
                + ", query-cache hits=" + queryCacheHitsAfterSecond
                + ", L2 entity-cache hits=" + l2EntityHitsAfterSecond
                + " -- contrary to the original article, the second run costs ZERO SQL statements: "
                + "the query cache stores the full row tuples, not just the 5 ids, and Hibernate "
                + "rebuilds UncachedProduct instances straight from that stored data. The zero L2 "
                + "entity-cache hits confirm this does not depend on UncachedProduct having its own "
                + "L2 region at all -- it has none.");

        assertThat(first).hasSize(5);
        assertThat(second).hasSize(5);
        assertThat(queriesForFirstRun)
                .as("the first, cold run executes the query itself as ONE statement")
                .isEqualTo(1);
        assertThat(queryCacheHitsAfterSecond)
                .as("the second run IS a genuine query-cache hit")
                .isEqualTo(1L);
        assertThat(queriesForSecondRun)
                .as("corrected finding: the second run costs ZERO SQL statements, not the 5 individual "
                        + "SELECTs the original article claimed -- the query cache reconstitutes the "
                        + "entities directly from its own stored tuple data")
                .isZero();
        assertThat(l2EntityHitsAfterSecond)
                .as("and none of that reconstruction is an L2 entity-cache hit -- UncachedProduct has "
                        + "no L2 region to hit, so the mechanism is the query cache's own stored data, "
                        + "not a secretly-enabled entity cache")
                .isZero();
    }
}