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What is a composite index, and does column order matter?
Databases & Caching · Intermediate level
Answer
A composite index covers multiple columns, and column order matters because the optimizer can use the leftmost prefix most effectively. Equality filters usually come before range or sort columns, but the final decision is based on the exact query and data distribution.
Technical explanation
Use real workload evidence from pg_stat_statements, slow logs, Performance Insights, or traces before adding indexes.
EXPLAIN shows the plan; EXPLAIN ANALYZE runs the query and compares estimated versus actual rows and timing.
Sequential scans are not always bad; for small tables or low-selectivity filters they may be optimal.
Hands-on example
Index tuning example:
EXPLAIN (ANALYZE, BUFFERS) SELECT * FROM orders WHERE customer_id = $1 ORDER BY created_at DESC LIMIT 20;
CREATE INDEX CONCURRENTLY idx_orders_customer_created ON orders(customer_id, created_at DESC);
Re-run EXPLAIN and confirm lower execution time, fewer buffers read, and no large sort. For covering reads, add INCLUDE columns where appropriate.
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