@Query for custom JPQL in Spring Boot - Performance & Optimization

Performance: @Query for custom JPQL

MEDIUM IMPACT

This affects database query execution time and how quickly data is fetched and rendered on the page.

Fetching filtered data from the database using JPQL

Spring Boot

@Query("SELECT u FROM User u WHERE u.status = :status")
List<User> findUsersByStatus(@Param("status") String status);

Filters data at the database level, reducing data size and speeding up response.

📈 Performance GainReduces data transfer size; lowers LCP by faster query execution.

Fetching filtered data from the database using JPQL

Spring Boot

@Query("SELECT u FROM User u")
List<User> findAllUsers();

Fetching all users without filtering causes large data transfer and slow response.

📉 Performance CostBlocks rendering until full data loads; increases LCP time significantly.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Fetching all data without filters	N/A (server-side)	N/A	High due to large data	[X] Bad
Fetching filtered data with JPQL @Query	N/A (server-side)	N/A	Lower due to smaller data	[✓] Good

Rendering Pipeline

The JPQL query runs on the server before the page renders. Efficient queries reduce server processing and data transfer, speeding up the browser's rendering.

→Server Processing

→Network Transfer

→Browser Rendering

⚠️ BottleneckServer Processing and Network Transfer due to large or inefficient queries

Core Web Vital Affected

LCP

This affects database query execution time and how quickly data is fetched and rendered on the page.

Optimization Tips

1Always filter data in JPQL queries to reduce response size.

2Avoid fetching unnecessary fields or records in @Query.

3Use pagination with JPQL to limit data per request.

Performance Quiz - 3 Questions

How does using a filtered JPQL @Query affect page load performance?

A It increases data size and slows down loading.

B It reduces data size and speeds up loading.

C It has no effect on performance.

D It blocks rendering indefinitely.

DevTools: Network

How to check: Open DevTools, go to Network tab, filter by XHR or Fetch, and observe the size and timing of API responses.

What to look for: Look for smaller response sizes and faster response times indicating efficient queries.

Practice

1. What is the main purpose of using @Query annotation in Spring Data JPA?

easy

A. To write custom JPQL or SQL queries when default methods are insufficient

B. To automatically generate database tables

C. To configure database connection properties

D. To define entity relationships

Solution

1. Step 1: Understand default query methods

   Spring Data JPA provides default query methods like findById, but they are limited.

2. Step 2: Role of @Query

   @Query allows writing custom JPQL or SQL queries to handle complex or specific data retrieval needs.

3. Final Answer:

   To write custom JPQL or SQL queries when default methods are insufficient -> Option A

4. Quick Check:

   @Query purpose = custom queries [OK]

Hint: Remember @Query is for custom queries beyond defaults [OK]

Common Mistakes:

• Thinking @Query creates tables
• Confusing @Query with database config
• Assuming @Query defines entity relations

2. Which of the following is the correct syntax to define a custom JPQL query using @Query in a Spring Data JPA repository interface?

easy

A. @Query("SELECT u FROM User u WHERE u.name = :name") List<User> findByName(@Param("name") String name);

B. @Query(SELECT * FROM User WHERE name = :name) List<User> findByName(String name);

C. @Query("SELECT * FROM User WHERE name = ?1") List<User> findByName(String name);

D. @Query("FROM User WHERE name = ?") List<User> findByName(@Param("name") String name);

Solution

1. Step 1: Check JPQL syntax

   JPQL uses entity names and fields, not table names or * syntax.

2. Step 2: Parameter binding

   Named parameters use :paramName and must be linked with @Param("paramName") in method.

3. Final Answer:

   @Query("SELECT u FROM User u WHERE u.name = :name") List<User> findByName(@Param("name") String name); -> Option A

4. Quick Check:

   JPQL + named param + @Param = correct syntax [OK]

Hint: Use entity names and :param with @Param for correct JPQL [OK]

Common Mistakes:

• Using SQL syntax (*) instead of JPQL
• Missing @Param annotation for named parameters
• Using positional parameters incorrectly

3. Given the repository method:

@Query("SELECT u FROM User u WHERE u.age > :minAge")
List<User> findUsersOlderThan(@Param("minAge") int minAge);

What will be the result of calling findUsersOlderThan(30)?

medium

A. A list of User entities with age less than 30

B. A list of User entities with age equal to 30

C. A list of User entities with age greater than 30

D. A runtime error due to missing parameter

Solution

1. Step 1: Analyze the JPQL query

   The query selects users where age is greater than the parameter minAge.

2. Step 2: Understand method call

   Calling findUsersOlderThan(30) sets minAge to 30, so users older than 30 are returned.

3. Final Answer:

   A list of User entities with age greater than 30 -> Option C

4. Quick Check:

   minAge=30, query > minAge = users older than 30 [OK]

Hint: Check parameter value and query condition carefully [OK]

Common Mistakes:

• Confusing > with >= or =
• Assuming parameter is ignored
• Expecting users younger than 30

4. Identify the error in the following repository method:

@Query("SELECT u FROM User u WHERE u.email = :email")
List<User> findByEmail(String email);

medium

A. JPQL query uses wrong entity name

B. Missing @Param annotation for the email parameter

C. Return type should be User, not List<User>

D. Query should use native SQL syntax

Solution

1. Step 1: Check parameter binding

   The query uses a named parameter :email, so the method parameter must have @Param("email") annotation.

2. Step 2: Validate other parts

   Entity name User is correct, return type List<User> is valid, and JPQL syntax is correct.

3. Final Answer:

   Missing @Param annotation for the email parameter -> Option B

4. Quick Check:

   Named param requires @Param annotation [OK]

Hint: Always add @Param for named parameters in @Query [OK]

Common Mistakes:

• Omitting @Param causes runtime errors
• Confusing JPQL with native SQL
• Assuming return type must be single entity

5. You want to write a custom JPQL query using @Query to find all users whose name contains a given substring (case insensitive). Which of the following method definitions correctly achieves this?

hard

A. @Query("SELECT u FROM User u WHERE u.name LIKE :namePart") List<User> findByNameLike(@Param("namePart") String namePart);

B. @Query("SELECT u FROM User u WHERE u.name LIKE '%:namePart%'") List<User> findByNameContains(@Param("namePart") String namePart);

C. @Query("SELECT u FROM User u WHERE u.name = :namePart") List<User> findByNameExact(@Param("namePart") String namePart);

D. @Query("SELECT u FROM User u WHERE LOWER(u.name) LIKE LOWER(CONCAT('%', :namePart, '%'))") List<User> findByNameContainsIgnoreCase(@Param("namePart") String namePart);

Solution

1. Step 1: Understand case insensitive search

   Use LOWER() on both field and parameter to ignore case.

2. Step 2: Use LIKE with wildcards

   Concatenate '%' before and after parameter to find substring matches.

3. Step 3: Check parameter binding

   Named parameter :namePart is linked with @Param("namePart") correctly.

4. Final Answer:

   @Query("SELECT u FROM User u WHERE LOWER(u.name) LIKE LOWER(CONCAT('%', :namePart, '%'))") List<User> findByNameContainsIgnoreCase(@Param("namePart") String namePart); -> Option D

5. Quick Check:

   LOWER + LIKE + CONCAT + @Param = correct case-insensitive contains [OK]

Hint: Use LOWER() and CONCAT('%', param, '%') for case-insensitive contains [OK]

Common Mistakes:

• Using LIKE with parameter inside quotes disables binding
• Not using LOWER() for case insensitivity
• Using = instead of LIKE for substring search