EXPLAIN Review Skill

## EXPLAIN Review Skill - Quick checklist
- [ ] EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT) attached
- [ ] Runs on staging with production-scale stats (ANALYZE recent)
- [ ] No unexpected Seq Scan on large tables
- [ ] Row estimates within 10x of actual (Planning vs Actual rows)
- [ ] No Sort on large rowsets without LIMIT index support
- [ ] Buffer read count justified for query frequency

EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT)
SELECT o.id, o.total
FROM orders o
WHERE o.tenant_id = $1 AND o.status = 'open'
ORDER BY o.created_at DESC
LIMIT 50;

EXPLAIN (ANALYZE, BUFFERS)
SELECT * FROM orders
WHERE tenant_id = $1 AND (status = 'open' OR status = 'pending');

FAIL signals in output:
  Seq Scan on orders  (cost=0..250000 rows=1200000)
  Buffers: shared read=180000
  Planning Rows: 50000  Actual Rows: 890000

## Verdict: FAIL
 
### Issues
1. Seq Scan - filter on tenant_id + status should use index
2. Row estimate off 18x - run ANALYZE orders
3. OR prevents simple btree composite use - consider IN (...) rewrite
 
### Suggested fix
```sql
SELECT * FROM orders
WHERE tenant_id = $1 AND status IN ('open', 'pending');

CREATE INDEX CONCURRENTLY idx_orders_tenant_status
  ON orders (tenant_id, status);

SELECT relname, last_analyze, last_autoanalyze
FROM pg_stat_user_tables WHERE relname = 'orders';
ANALYZE orders;  -- staging before EXPLAIN if stale