Loading trivial aggregates with Single Query Loading.

Since trivial aggregates consist only of a single table this doesn't make a real difference,
but it demonstrates that the general infrastructure works.

See #1446

spring-data-jdbc/pom.xml

@@ -191,6 +191,12 @@
 
 		<!-- Testcontainers -->
 
+		<dependency>
+			<groupId>com.github.jsqlparser</groupId>
+			<artifactId>jsqlparser</artifactId>
+			<version>4.5</version>
+		</dependency>
+
 		<dependency>
 			<groupId>org.testcontainers</groupId>
 			<artifactId>mysql</artifactId>

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/convert/AggregateReader.java

@@ -0,0 +1,126 @@
+/*
+ * Copyright 2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.data.jdbc.core.convert;
+
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+
+import org.springframework.dao.IncorrectResultSizeDataAccessException;
+import org.springframework.data.jdbc.core.convert.sqlgeneration.AliasFactory;
+import org.springframework.data.jdbc.core.convert.sqlgeneration.SingleQuerySqlGenerator;
+import org.springframework.data.mapping.PersistentPropertyPath;
+import org.springframework.data.relational.core.dialect.Dialect;
+import org.springframework.data.relational.core.mapping.PersistentPropertyPathExtension;
+import org.springframework.data.relational.core.mapping.RelationalMappingContext;
+import org.springframework.data.relational.core.mapping.RelationalPersistentEntity;
+import org.springframework.data.relational.core.mapping.RelationalPersistentProperty;
+import org.springframework.jdbc.core.namedparam.NamedParameterJdbcOperations;
+import org.springframework.util.Assert;
+
+public class AggregateReader<T> {
+
+	private final RelationalMappingContext mappingContext;
+	private final RelationalPersistentEntity<T> aggregate;
+	private final AliasFactory aliasFactory = new AliasFactory();
+	private final SingleQuerySqlGenerator sqlGenerator;
+	private final JdbcConverter converter;
+	private final NamedParameterJdbcOperations jdbcTemplate;
+
+	AggregateReader(RelationalMappingContext mappingContext, Dialect dialect, JdbcConverter converter,
+			NamedParameterJdbcOperations jdbcTemplate, RelationalPersistentEntity<T> aggregate) {
+
+		this.mappingContext = mappingContext;
+
+		this.aggregate = aggregate;
+		this.converter = converter;
+		this.jdbcTemplate = jdbcTemplate;
+
+		this.sqlGenerator = new SingleQuerySqlGenerator(mappingContext, dialect, aggregate);
+	}
+
+	public List<T> findAll() {
+
+		String sql = sqlGenerator.findAll();
+
+		PathToColumnMapping pathToColumn = createPathToColumnMapping(aliasFactory);
+		AggregateResultSetExtractor<T> extractor = new AggregateResultSetExtractor<>(mappingContext, aggregate, converter,
+				pathToColumn);
+
+		Iterable<T> result = jdbcTemplate.query(sql, extractor);
+
+		return (List<T>) result;
+	}
+
+	public T findById(Object id) {
+
+		PathToColumnMapping pathToColumn = createPathToColumnMapping(aliasFactory);
+		AggregateResultSetExtractor<T> extractor = new AggregateResultSetExtractor<>(mappingContext, aggregate, converter,
+				pathToColumn);
+
+		String sql = sqlGenerator.findById();
+
+		id = converter.writeValue(id, aggregate.getIdProperty().getTypeInformation());
+
+		Iterator<T> result = jdbcTemplate.query(sql, Map.of("id", id), extractor).iterator();
+
+		T returnValue = result.hasNext() ? result.next() : null;
+
+		if (result.hasNext()) {
+			throw new IncorrectResultSizeDataAccessException(1);
+		}
+
+		return returnValue;
+	}
+
+	public Iterable<T> findAllById(Iterable<?> ids) {
+
+		PathToColumnMapping pathToColumn = createPathToColumnMapping(aliasFactory);
+		AggregateResultSetExtractor<T> extractor = new AggregateResultSetExtractor<>(mappingContext, aggregate, converter,
+				pathToColumn);
+
+		String sql = sqlGenerator.findAllById();
+
+		List<Object> convertedIds = new ArrayList<>();
+		for (Object id : ids) {
+			convertedIds.add(converter.writeValue(id, aggregate.getIdProperty().getTypeInformation()));
+		}
+
+		return jdbcTemplate.query(sql, Map.of("ids", convertedIds), extractor);
+	}
+
+	private PathToColumnMapping createPathToColumnMapping(AliasFactory aliasFactory) {
+		return new PathToColumnMapping() {
+			@Override
+			public String column(PersistentPropertyPath<RelationalPersistentProperty> propertyPath) {
+
+				PersistentPropertyPathExtension aliasLookUpKey = new PersistentPropertyPathExtension(mappingContext, propertyPath);
+
+				String alias = aliasFactory.getAlias(aliasLookUpKey);
+				Assert.notNull(alias, () -> "alias for >" + aliasLookUpKey + "<must not be null");
+				return alias;
+			}
+
+			@Override
+			public String keyColumn(PersistentPropertyPath<RelationalPersistentProperty> propertyPath) {
+				return aliasFactory.getKeyAlias(propertyPath);
+			}
+		};
+	}
+
+}

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/core/convert/AggregateReaderFactory.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2023 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.data.jdbc.core.convert;
+
+import java.util.HashMap;
+
+import org.springframework.data.relational.core.dialect.Dialect;
+import org.springframework.data.relational.core.mapping.RelationalMappingContext;
+import org.springframework.data.relational.core.mapping.RelationalPersistentEntity;
+import org.springframework.jdbc.core.namedparam.NamedParameterJdbcOperations;
+
+public class AggregateReaderFactory {
+
+	private final RelationalMappingContext mappingContext;
+	private final Dialect dialect;
+	private final JdbcConverter converter;
+	private final NamedParameterJdbcOperations jdbcTemplate;
+
+	private final HashMap<RelationalPersistentEntity<?>, AggregateReader> cash = new HashMap<>();
+
+	public AggregateReaderFactory(RelationalMappingContext mappingContext, Dialect dialect, JdbcConverter converter,
+			NamedParameterJdbcOperations jdbcTemplate) {
+		this.mappingContext = mappingContext;
+		this.dialect = dialect;
+		this.converter = converter;
+		this.jdbcTemplate = jdbcTemplate;
+	}
+
+	<T> AggregateReader<T> createAggregateReaderFor(RelationalPersistentEntity<T> entity) {
+		return cash.computeIfAbsent(entity, e -> new AggregateReader<>(mappingContext, dialect, converter, jdbcTemplate, e));
+	}
+}