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What does an execution plan tell you, and what is a sequential scan versus an index scan?
Databases & Caching · Intermediate level
Answer
An execution plan shows how the database will access, join, sort, and filter data. A sequential scan reads the table broadly; an index scan uses an index to find matching rows. The right choice depends on table size, selectivity, and cost.
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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