Limit single finders max results to 2 for performance.

Closes spring-projects#2594
Original pull request spring-projects#2604

Author: quaff

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/support/QuerydslJpaPredicateExecutor.java

@@ -91,7 +91,7 @@ public Optional<T> findOne(Predicate predicate) {
 		Assert.notNull(predicate, "Predicate must not be null");
 
 		try {
-			return Optional.ofNullable(createQuery(predicate).select(path).fetchOne());
+			return Optional.ofNullable(createQuery(predicate).select(path).limit(2).fetchOne());
 		} catch (NonUniqueResultException ex) {
 			throw new IncorrectResultSizeDataAccessException(ex.getMessage(), 1, ex);
 		}

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/support/QuerydslJpaRepository.java

@@ -101,7 +101,7 @@ public QuerydslJpaRepository(JpaEntityInformation<T, ID> entityInformation, Enti
 	public Optional<T> findOne(Predicate predicate) {
 
 		try {
-			return Optional.ofNullable(createQuery(predicate).select(path).fetchOne());
+			return Optional.ofNullable(createQuery(predicate).select(path).limit(2).fetchOne());
 		} catch (NonUniqueResultException ex) {
 			throw new IncorrectResultSizeDataAccessException(ex.getMessage(), 1, ex);
 		}

spring-data-jpa/src/main/java/org/springframework/data/jpa/repository/support/SimpleJpaRepository.java

@@ -452,7 +452,7 @@ public Page<T> findAll(Pageable pageable) {
 	public Optional<T> findOne(@Nullable Specification<T> spec) {
 
 		try {
-			return Optional.of(getQuery(spec, Sort.unsorted()).getSingleResult());
+			return Optional.of(getQuery(spec, Sort.unsorted()).setMaxResults(2).getSingleResult());
 		} catch (NoResultException e) {
 			return Optional.empty();
 		}
@@ -482,7 +482,7 @@ public <S extends T> Optional<S> findOne(Example<S> example) {
 		try {
 			return Optional
 					.of(getQuery(new ExampleSpecification<>(example, escapeCharacter), example.getProbeType(), Sort.unsorted())
-							.getSingleResult());
+							.setMaxResults(2).getSingleResult());
 		} catch (NoResultException e) {
 			return Optional.empty();
 		}