Introduction

When building backend applications, performance bottlenecks often hide in unexpected places.
One such issue I encountered was duplicate database queries executed inside loops.

The Problem

• Symptom: Application response time was slow, especially with larger datasets.
• Root cause: For every iteration in a loop, a new query was fired to the database.
• Impact:
  • High database load.
  • Slow performance.
  • Poor scalability as data volume increased.

The Solution: Bulk Fetch + Map Lookup

Instead of querying the database inside the loop, I :

1. Fetched all required records in a single bulk query.
2. Stored the results in a Map (key-value structure) for O(1) lookups.
3. Replaced repeated queries with fast in-memory lookups.

⚡ Result: Performance improved drastically, reducing database calls by up to 90% in some cases.

Sample Code Snippets

❌ Bad Approach (Query inside loop)

for (Order order : orders) { 
    Customer customer = customerRepository.findById(order.getCustomerId());  
    // Runs a DB query for every order! 
    process(order, customer); 
} 

✅ Optimized Approach (Bulk Fetch + Map)

List<Customer> customers = customerRepository.findAllById( 
    orders.stream().map(Order::getCustomerId).collect(Collectors.toList()) 
); 
// Convert to Map for fast lookup 
Map<Long, Customer> customerMap = customers.stream() 
    .collect(Collectors.toMap(Customer::getId, Function.identity())); 
// Use in loop without extra DB calls 
for (Order order : orders) { 
    Customer customer = customerMap.get(order.getCustomerId()); 
    process(order, customer); 
} 

SQL Perspective (Bulk Fetch)

Instead of this (inefficient):

SELECT * FROM customers WHERE id = 1; 
SELECT * FROM customers WHERE id = 2; 
SELECT * FROM customers WHERE id = 3; 

We now use a single bulk query:

SELECT * FROM customers WHERE id IN (1, 2, 3, ...); 

Benefits Achieved

✅ Reduced redundant database queries
✅ Improved performance and scalability
✅ Lower database load
✅ Cleaner and more maintainable code

Best Practices

• Always check if your loop is triggering repeated queries.
• Prefer bulk fetching with IN clauses or joins.
• Use maps/dictionaries for lookups in memory.
• Profile and monitor query counts in your ORM (e.g., Hibernate/JPA SQL logs).

Conclusion

Eliminating duplicate queries inside loops may seem like a small optimization, but it can have a huge impact on performance. By moving from multiple individual queries to bulk fetching with a lookup map, I was able to:

• Reduce database load,
• Speed up response times,
• And make the system scale much better under heavy data loads.

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