Use EXPLAIN ANALYZE to Diagnose Slow Queries#
EXPLAIN ANALYZE executes the query and shows actual timings, revealing the true performance bottlenecks.
Incorrect (guessing at performance issues):
-- Query is slow, but why?
select * from orders where customer_id = 123 and status = 'pending';
-- "It must be missing an index" - but which one?
Correct (use EXPLAIN ANALYZE):
explain (analyze, buffers, format text)
select * from orders where customer_id = 123 and status = 'pending';
-- Output reveals the issue:
-- Seq Scan on orders (cost=0.00..25000.00 rows=50 width=100) (actual time=0.015..450.123 rows=50 loops=1)
-- Filter: ((customer_id = 123) AND (status = 'pending'::text))
-- Rows Removed by Filter: 999950
-- Buffers: shared hit=5000 read=15000
-- Planning Time: 0.150 ms
-- Execution Time: 450.500 ms
Key things to look for:
-- Seq Scan on large tables = missing index
-- Rows Removed by Filter = poor selectivity or missing index
-- Buffers: read >> hit = data not cached, needs more memory
-- Nested Loop with high loops = consider different join strategy
-- Sort Method: external merge = work_mem too low
Reference: EXPLAIN