diff --git a/src/main/java/com/example/oneul/domain/post/dao/PostCommandRepository.java b/src/main/java/com/example/oneul/domain/post/dao/PostCommandRepository.java
index df1745b..5e9f9df 100644
--- a/src/main/java/com/example/oneul/domain/post/dao/PostCommandRepository.java
+++ b/src/main/java/com/example/oneul/domain/post/dao/PostCommandRepository.java
@@ -12,4 +12,4 @@ public interface PostCommandRepository extends CrudRepository<Post, Long> {
     Optional<Post> findByIdAndWriter(Long id, UserEntity writer);
     void deleteById(Long id);
     void delete(Post post);
-}
+}
\ No newline at end of file
diff --git a/src/main/java/com/example/oneul/domain/post/domain/Post.java b/src/main/java/com/example/oneul/domain/post/domain/Post.java
index 6e13b8a..61537f8 100644
--- a/src/main/java/com/example/oneul/domain/post/domain/Post.java
+++ b/src/main/java/com/example/oneul/domain/post/domain/Post.java
@@ -12,7 +12,9 @@ import javax.persistence.FetchType;
 import javax.persistence.GeneratedValue;
 import javax.persistence.GenerationType;
 import javax.persistence.Id;
+import javax.persistence.Index;
 import javax.persistence.ManyToOne;
+import javax.persistence.Table;
 
 import com.example.oneul.domain.user.domain.UserEntity;
 
@@ -21,6 +23,7 @@ import org.springframework.data.jpa.domain.support.AuditingEntityListener;
 
 @Entity
 @EntityListeners(AuditingEntityListener.class)
+@Table(indexes = @Index(name = "i_post", columnList="createdAt"))
 public class Post {
     @Id @GeneratedValue(strategy = GenerationType.AUTO)
     private Long id;
diff --git a/src/main/java/com/example/oneul/domain/user/dao/UserRepository.java b/src/main/java/com/example/oneul/domain/user/dao/UserRepository.java
index 87f4867..7d1c1a5 100644
--- a/src/main/java/com/example/oneul/domain/user/dao/UserRepository.java
+++ b/src/main/java/com/example/oneul/domain/user/dao/UserRepository.java
@@ -4,10 +4,12 @@ import java.util.Optional;
 
 import com.example.oneul.domain.user.domain.UserEntity;
 
+import org.springframework.dao.DataIntegrityViolationException;
 import org.springframework.data.repository.CrudRepository;
 import org.springframework.stereotype.Repository;
 
 @Repository
 public interface UserRepository extends CrudRepository<UserEntity, Long> {
     Optional<UserEntity> findByUsername(String username);
+    UserEntity save(UserEntity userEntity) throws DataIntegrityViolationException;
 }
diff --git a/src/main/java/com/example/oneul/domain/user/service/UserServiceImpl.java b/src/main/java/com/example/oneul/domain/user/service/UserServiceImpl.java
index 7cfed21..a062f1b 100644
--- a/src/main/java/com/example/oneul/domain/user/service/UserServiceImpl.java
+++ b/src/main/java/com/example/oneul/domain/user/service/UserServiceImpl.java
@@ -4,6 +4,7 @@ import javax.servlet.http.HttpSession;
 
 import com.example.oneul.domain.user.dao.UserRepository;
 import com.example.oneul.domain.user.domain.UserEntity;
+import com.example.oneul.domain.user.exception.UserAlreadyExistException;
 import com.example.oneul.domain.user.exception.WrongUsernameAndPasswordException;
 
 import org.slf4j.Logger;
@@ -28,11 +29,15 @@ public class UserServiceImpl implements UserService {
     @Override
     @Transactional(isolation = Isolation.SERIALIZABLE)
     public UserEntity signUp(UserEntity userEntity){
+        if(userRepository.findByUsername(userEntity.getUsername()).isPresent()){
+            throw new UserAlreadyExistException(userEntity.getUsername() + " is already exists");
+        }
+        
         UserEntity user = UserEntity.builder().username(userEntity.getUsername())
                                               .password(
                                                   passwordEncoder.encode(userEntity.getPassword()))
                                               .build();
-        return userRepository.save(user);
+        return user;
     }
 
     @Override
diff --git a/src/main/java/com/example/oneul/global/error/GlobalExceptionHandler.java b/src/main/java/com/example/oneul/global/error/GlobalExceptionHandler.java
index 4b9bd08..0fc6753 100644
--- a/src/main/java/com/example/oneul/global/error/GlobalExceptionHandler.java
+++ b/src/main/java/com/example/oneul/global/error/GlobalExceptionHandler.java
@@ -14,12 +14,6 @@ import org.springframework.web.bind.annotation.ExceptionHandler;
 public class GlobalExceptionHandler {
     private static final Logger log = LoggerFactory.getLogger(GlobalExceptionHandler.class);
 
-    @ExceptionHandler(Exception.class)
-    protected ResponseEntity<String> handleException(Exception e){
-        log.info(e.getMessage());
-        return new ResponseEntity<>("invalid request", HttpStatus.BAD_REQUEST);
-    }
-
     @ExceptionHandler(UserAlreadyExistException.class)
     protected ResponseEntity<String> handleUserAlreadyExistException(UserAlreadyExistException e){
         log.info(e.getMessage());
diff --git a/src/test/java/com/example/oneul/controller/UserControllerTest.java b/src/test/java/com/example/oneul/controller/UserControllerTest.java
index 2d20e8b..9890d1d 100644
--- a/src/test/java/com/example/oneul/controller/UserControllerTest.java
+++ b/src/test/java/com/example/oneul/controller/UserControllerTest.java
@@ -35,9 +35,10 @@ public class UserControllerTest {
     @Autowired
     private UserService userService;
     private MockHttpSession httpSession = new MockHttpSession();
+    private UserEntity testUser;
 
     @BeforeEach
-    public void setUp(){
+    public void eachSetUp(){
         mvc = MockMvcBuilders.standaloneSetup(new UserApi(userService))
             .addFilters(new CharacterEncodingFilter("UTF-8", true))
             .build();
@@ -53,10 +54,12 @@ public class UserControllerTest {
     }
     
     private UserEntity createTestUser(String username, String password){
-        return userService.signUp(UserEntity.builder()
+        if(testUser != null)
+            testUser =  userService.signUp(UserEntity.builder()
                                             .username(username)
                                             .password(password)
                                             .build());
+        return testUser;
     }
 
     @Test