How do you use EXPLAIN and EXPLAIN ANALYZE to debug pgvector queries?

-- Basic EXPLAIN: shows the plan without running the query
EXPLAIN
SELECT id, content FROM documents
ORDER BY embedding <=> '[0.1,0.2,0.3]'
LIMIT 5;

-- Sample output (with HNSW index):
-- Limit  (cost=...)
--   ->  Index Scan using documents_embedding_idx on documents
--         Order By: (embedding <=> '[0.1,0.2,0.3]'::vector)
-- This shows the index IS being used.

-- Sample output (sequential scan - index not used):
-- Limit  (cost=...)
--   ->  Sort  (cost=...)
--         Sort Key: ((embedding <=> '[0.1,0.2,0.3]'::vector))
--         ->  Seq Scan on documents
-- This is slow - investigate why the index was not used!

-- EXPLAIN ANALYZE: actually runs the query and shows real timings
EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT)
SELECT id FROM documents
WHERE category = 'tech'
ORDER BY embedding <=> '[...]' LIMIT 5;

-- Key metrics to look at in EXPLAIN ANALYZE:
-- 'Seq Scan' vs 'Index Scan' - index should be used for KNN
-- actual rows vs estimated rows - large discrepancy = stale stats
-- actual time - how long each node actually took
-- Buffers: hit=N miss=N - cache hit ratio

-- Force sequential scan to compare:
SET enable_indexscan = off;
EXPLAIN ANALYZE SELECT id FROM documents ORDER BY embedding <=> '[...]' LIMIT 5;
RESET enable_indexscan;

-- Update statistics if planner makes poor decisions:
ANALYZE documents;  -- refresh table statistics