---

name: database-performance-debugging description: Debug database performance issues through query analysis, index optimization, and execution plan review. Identify and fix slow queries.

Database Performance Debugging

Overview

Database performance issues directly impact application responsiveness. Debugging focuses on identifying slow queries and optimizing execution plans.

When to Use

• Slow application response times
• High database CPU
• Slow queries identified
• Performance regression
• Under load stress

Instructions

1. Identify Slow Queries

-- Enable slow query log (MySQL)
SET GLOBAL slow_query_log = 'ON';
SET GLOBAL long_query_time = 0.5;

-- View slow queries
SHOW GLOBAL STATUS LIKE 'Slow_queries';
SELECT * FROM mysql.slow_log;

-- PostgreSQL slow queries
CREATE EXTENSION pg_stat_statements;
SELECT mean_exec_time, calls, query
FROM pg_stat_statements
ORDER BY mean_exec_time DESC LIMIT 10;

-- SQL Server slow queries
SELECT TOP 10
  execution_count,
  total_elapsed_time,
  statement_text
FROM sys.dm_exec_query_stats
ORDER BY total_elapsed_time DESC;

-- Query profiling
EXPLAIN ANALYZE
SELECT * FROM orders WHERE user_id = 123;
-- Slow: Seq Scan (full table scan)
-- Fast: Index Scan

2. Common Issues & Solutions

Issue: N+1 Query Problem

Symptom: 1001 queries for 1000 records

Example (Python):
  for user in users:
    posts = db.query(Post).filter(Post.user_id == user.id)
    # 1 + 1000 queries

Solution:
  users = db.query(User).options(joinedload(User.posts))
  # Single query with JOIN

---

Issue: Missing Index

Symptom: Seq Scan instead of Index Scan

Solution:
  CREATE INDEX idx_orders_user_id ON orders(user_id);
  Verify: EXPLAIN ANALYZE shows Index Scan now

---

Issue: Inefficient JOIN

Before:
  SELECT * FROM orders o, users u
  WHERE o.user_id = u.id AND u.email LIKE '%@example.com'
  # Bad: Table scan on users for every order

After:
  SELECT o.* FROM orders o
  JOIN users u ON o.user_id = u.id
  WHERE u.email = 'exact@example.com'
  # Good: Single email lookup

---

Issue: Large Table Scan

Symptom: SELECT * FROM large_table (1M rows)

Solutions:
  1. Add LIMIT clause
  2. Add WHERE condition
  3. Select specific columns
  4. Use pagination
  5. Archive old data

---

Issue: Slow Aggregation

Before (1 minute):
  SELECT user_id, COUNT(*), SUM(amount)
  FROM transactions
  GROUP BY user_id

After (50ms):
  SELECT user_id, transaction_count, total_amount
  FROM user_transaction_stats
  WHERE updated_at > NOW() - INTERVAL 1 DAY
  # Materialized view or aggregation table

3. Execution Plan Analysis

EXPLAIN Output Understanding:

Seq Scan (Full Table Scan):
  - Reads entire table
  - Slowest method
  - Fix: Add index

Index Scan:
  - Uses index
  - Fast
  - Ideal

Bitmap Index Scan:
  - Partial index scan
  - Converts to heap scan
  - Moderate speed

Nested Loop:
  - For each row in left, scan right
  - O(n*m) complexity
  - Slow for large tables

Hash Join:
  - Build hash table of smaller table
  - Probe with larger table
  - Faster than nested loop

Merge Join:
  - Sort both tables, merge
  - Fastest for large sorted data
  - Requires sort operation

---

Reading EXPLAIN ANALYZE:

Node: Seq Scan on orders (actual 8023.456 ms)
  - Seq Scan = Full table scan
  - actual time = real execution time
  - 8023 ms = TOO SLOW

Rows: 1000000 (estimated) 1000000 (actual)
  - Match = planner accurate
  - Mismatch = update statistics

Node: Index Scan (actual 15.234 ms)
  - Index Scan = Fast
  - 15 ms = ACCEPTABLE

4. Debugging Process

Steps:

1. Identify Slow Query
  - Enable slow query logging
  - Run workload
  - Review slow log
  - Note execution time

2. Analyze with EXPLAIN
  - Run EXPLAIN ANALYZE
  - Look for Seq Scan
  - Check estimated vs actual rows
  - Review join methods

3. Find Root Cause
  - Missing index?
  - Inefficient join?
  - Missing WHERE clause?
  - Outdated statistics?

4. Try Fix
  - Add index
  - Rewrite query
  - Update statistics
  - Archive old data

5. Measure Improvement
  - Run query after fix
  - Compare execution time
  - Before: 5000ms
  - After: 100ms (50x faster!)

6. Monitor
  - Track slow queries
  - Set baseline
  - Alert on regression
  - Periodic review

---

Checklist:

[ ] Slow query identified and logged
[ ] EXPLAIN ANALYZE run
[ ] Estimated vs actual rows analyzed
[ ] Seq Scans identified
[ ] Indexes checked
[ ] Join strategy reviewed
[ ] Statistics updated
[ ] Query rewritten if needed
[ ] Index created if needed
[ ] Fix verified
[ ] Performance baseline established
[ ] Monitoring configured
[ ] Documented for team

Key Points

• Enable slow query logging in production
• Use EXPLAIN ANALYZE to investigate
• Look for Seq Scan = missing index
• Add indexes to WHERE/JOIN columns
• Monitor query statistics
• Update table statistics regularly
• Rewrite queries to avoid inefficiencies
• Use pagination for large result sets
• Measure before and after optimization
• Track slow query trends