Monitor explain analyze

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