You have a complex query that runs slowly in your Spring Boot application. How would you optimize it?

Question

Accepted Answer

Optimizing a slow complex query in a Spring Boot application starts with profiling to locate the actual bottleneck, then addresses it at the query, schema, and application layers rather than guessing at a fix.

Key Points:
• Use database profiling tools like EXPLAIN ANALYZE to identify missing indexes, expensive joins, or full table scans.
• Rewrite the query to reduce unnecessary joins, select only needed columns, or split it into simpler steps.
• Add or adjust indexes on columns used in WHERE, JOIN, and ORDER BY clauses based on the query plan.
• Apply application-level caching with @Cacheable for frequently requested, rarely changing data to avoid repeated database hits.
• Consider pagination for large result sets instead of loading everything into memory at once.

Example:
Running EXPLAIN ANALYZE on a slow order-history query reveals a full table scan; adding a composite index on (customer_id, created_at) cuts the query time from several seconds to milliseconds.

Code Example:
```
@Cacheable("productCatalog")
public List getFeaturedProducts() {
    return productRepository.findFeatured();
}
```

Interview Tip:
A concise interview answer is:

"I'd start by profiling the query with EXPLAIN ANALYZE to find the actual bottleneck, then fix it at the source, whether that's adding an index, reducing joins, or restructuring the query. If it's read-heavy and the data doesn't change often, I'd also add caching with @Cacheable to cut down repeated database hits."