Database Performance Investigation Runbook#

When a database is slow, resist the urge to immediately tune configuration parameters. Follow this sequence: identify what is slow, understand why, then fix the specific bottleneck. Most performance problems are caused by missing indexes or a single bad query, not global configuration issues.

Phase 1 – Identify Slow Queries#

The first step is always finding which queries are consuming the most time.

PostgreSQL: pg_stat_statements#

Enable the extension if not already loaded:

PostgreSQL Performance Tuning#

Most PostgreSQL performance problems come from missing indexes, bad query plans, connection overhead, or table bloat. This covers how to diagnose each one.

Reading EXPLAIN ANALYZE#

EXPLAIN shows the query plan. EXPLAIN ANALYZE actually executes the query and shows real timings.

EXPLAIN ANALYZE SELECT * FROM orders WHERE customer_id = 42 AND status = 'pending';

Index Scan using idx_orders_customer on orders  (cost=0.43..8.45 rows=1 width=120) (actual time=0.023..0.025 rows=3 loops=1)
  Index Cond: (customer_id = 42)
  Filter: (status = 'pending'::text)
  Rows Removed by Filter: 12
Planning Time: 0.152 ms
Execution Time: 0.048 ms

What to look for: Seq Scan on large tables means a missing index. Rows Removed by Filter means the index fetched extra rows that a composite index would eliminate. actual rows far from estimated rows means stale statistics – run ANALYZE tablename;. Nested Loop with high loops count usually wants a hash join; check the inner table’s indexes.