DATAJPA-477 - Avoid query creation in PagedExecution if count query returns 0.

We now skip the query creation if we detect that the preceding count query returned 0 or no results. Previously we always created a query in every case even if it was not necessary.

Original pull request: #113.

src/main/java/org/springframework/data/jpa/repository/query/JpaQueryExecution.java

@@ -175,10 +175,15 @@ public abstract class JpaQueryExecution {
 			List<Long> totals = projection.getResultList();
 			Long total = totals.size() == 1 ? totals.get(0) : totals.size();
 
-			Query query = repositoryQuery.createQuery(values);
 			ParameterAccessor accessor = new ParametersParameterAccessor(parameters, values);
 			Pageable pageable = accessor.getPageable();
 
+			if (total.equals(0L)) {
+				return new PageImpl<Object>(Collections.emptyList(), pageable, total);
+			}
+
+			Query query = repositoryQuery.createQuery(values);
+
 			List<Object> content = pageable == null || total > pageable.getOffset() ? query.getResultList() : Collections
 					.emptyList();
 

src/test/java/org/springframework/data/jpa/repository/query/JpaQueryExecutionUnitTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2008-2013 the original author or authors.
+ * Copyright 2008-2014 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.
@@ -17,6 +17,7 @@ package org.springframework.data.jpa.repository.query;
 
 import static org.hamcrest.CoreMatchers.*;
 import static org.junit.Assert.*;
+import static org.mockito.Matchers.*;
 import static org.mockito.Mockito.*;
 
 import java.util.Arrays;
@@ -41,6 +42,7 @@ import org.springframework.data.repository.query.Parameters;
  * Unit test for {@link JpaQueryExecution}.
  * 
  * @author Oliver Gierke
+ * @author Thomas Darimont
  */
 @RunWith(MockitoJUnitRunner.class)
 public class JpaQueryExecutionUnitTests {
@@ -128,6 +130,23 @@ public class JpaQueryExecutionUnitTests {
 		verify(query, times(0)).getResultList();
 	}
 
+	/**
+	 * @see DATAJPA-477
+	 */
+	@Test
+	public void pagedExecutionShouldNotGenerateUnecessaryQueryIfCountReportedNoResults() throws Exception {
+
+		Parameters<?, ?> parameters = new DefaultParameters(getClass().getMethod("sampleMethod", Pageable.class));
+		when(jpaQuery.createCountQuery(Mockito.any(Object[].class))).thenReturn(countQuery);
+		when(countQuery.getResultList()).thenReturn(Arrays.asList(0L));
+
+		PagedExecution execution = new PagedExecution(parameters);
+		execution.doExecute(jpaQuery, new Object[] { new PageRequest(0, 10) });
+
+		verify(query, times(0)).getResultList();
+		verify(jpaQuery, times(0)).createQuery((Object[]) any());
+	}
+
 	public static void sampleMethod(Pageable pageable) {
 
 	}