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a) Index Usage
   - Are WHERE columns indexed?
   - Are JOIN columns indexed?
   - Is the index type appropriate (B-tree, GIN, BRIN)?

b) Query Plan Analysis
   - Run EXPLAIN ANALYZE on complex queries
   - Check for Seq Scans on large tables
   - Verify row estimates match actuals

c) Common Issues
   - N+1 query patterns
   - Missing composite indexes
   - Wrong column order in indexes

a) Data Types
   - bigint for IDs (not int)
   - text for strings (not varchar(n) unless constraint needed)
   - timestamptz for timestamps (not timestamp)
   - numeric for money (not float)
   - boolean for flags (not varchar)

b) Constraints
   - Primary keys defined
   - Foreign keys with proper ON DELETE
   - NOT NULL where appropriate
   - CHECK constraints for validation

c) Naming
   - lowercase_snake_case (avoid quoted identifiers)
   - Consistent naming patterns

a) Row Level Security
   - RLS enabled on multi-tenant tables?
   - Policies use (select auth.uid()) pattern?
   - RLS columns indexed?

b) Permissions
   - Least privilege principle followed?
   - No GRANT ALL to application users?
   - Public schema permissions revoked?

c) Data Protection
   - Sensitive data encrypted?
   - PII access logged?

-- ❌ BAD: No index on foreign key
CREATE TABLE orders (
  id bigint PRIMARY KEY,
  customer_id bigint REFERENCES customers(id)
  -- Missing index!
);

-- ✅ GOOD: Index on foreign key
CREATE TABLE orders (
  id bigint PRIMARY KEY,
  customer_id bigint REFERENCES customers(id)
);
CREATE INDEX orders_customer_id_idx ON orders (customer_id);

-- ❌ BAD: B-tree for JSONB containment
CREATE INDEX products_attrs_idx ON products (attributes);
SELECT * FROM products WHERE attributes @> '{"color": "red"}';

-- ✅ GOOD: GIN for JSONB
CREATE INDEX products_attrs_idx ON products USING gin (attributes);

-- ❌ BAD: Separate indexes
CREATE INDEX orders_status_idx ON orders (status);
CREATE INDEX orders_created_idx ON orders (created_at);

-- ✅ GOOD: Composite index (equality columns first, then range)
CREATE INDEX orders_status_created_idx ON orders (status, created_at);

-- ❌ BAD: Must fetch name from table
CREATE INDEX users_email_idx ON users (email);
SELECT email, name FROM users WHERE email = 'user@example.com';

-- ✅ GOOD: All columns in index
CREATE INDEX users_email_idx ON users (email) INCLUDE (name, created_at);

-- ❌ BAD: Full index includes deleted rows
CREATE INDEX users_email_idx ON users (email);

-- ✅ GOOD: Partial index excludes deleted rows
CREATE INDEX users_active_email_idx ON users (email) WHERE deleted_at IS NULL;

-- ❌ BAD: Poor type choices
CREATE TABLE users (
  id int,                           -- Overflows at 2.1B
  email varchar(255),               -- Artificial limit
  created_at timestamp,             -- No timezone
  is_active varchar(5),             -- Should be boolean
  balance float                     -- Precision loss
);

-- ✅ GOOD: Proper types
CREATE TABLE users (
  id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
  email text NOT NULL,
  created_at timestamptz DEFAULT now(),
  is_active boolean DEFAULT true,
  balance numeric(10,2)
);

-- ✅ Single database: IDENTITY (default, recommended)
CREATE TABLE users (
  id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY
);

-- ✅ Distributed systems: UUIDv7 (time-ordered)
CREATE EXTENSION IF NOT EXISTS pg_uuidv7;
CREATE TABLE orders (
  id uuid DEFAULT uuid_generate_v7() PRIMARY KEY
);

-- ❌ AVOID: Random UUIDs cause index fragmentation
CREATE TABLE events (
  id uuid DEFAULT gen_random_uuid() PRIMARY KEY  -- Fragmented inserts!
);

-- ✅ GOOD: Partitioned by month
CREATE TABLE events (
  id bigint GENERATED ALWAYS AS IDENTITY,
  created_at timestamptz NOT NULL,
  data jsonb
) PARTITION BY RANGE (created_at);

CREATE TABLE events_2024_01 PARTITION OF events
  FOR VALUES FROM ('2024-01-01') TO ('2024-02-01');

CREATE TABLE events_2024_02 PARTITION OF events
  FOR VALUES FROM ('2024-02-01') TO ('2024-03-01');

-- Drop old data instantly
DROP TABLE events_2023_01;  -- Instant vs DELETE taking hours

-- ❌ BAD: Quoted mixed-case requires quotes everywhere
CREATE TABLE "Users" ("userId" bigint, "firstName" text);
SELECT "firstName" FROM "Users";  -- Must quote!

-- ✅ GOOD: Lowercase works without quotes
CREATE TABLE users (user_id bigint, first_name text);
SELECT first_name FROM users;

-- ❌ BAD: Application-only filtering
SELECT * FROM orders WHERE user_id = $current_user_id;
-- Bug means all orders exposed!

-- ✅ GOOD: Database-enforced RLS
ALTER TABLE orders ENABLE ROW LEVEL SECURITY;
ALTER TABLE orders FORCE ROW LEVEL SECURITY;

CREATE POLICY orders_user_policy ON orders
  FOR ALL
  USING (user_id = current_setting('app.current_user_id')::bigint);

-- Supabase pattern
CREATE POLICY orders_user_policy ON orders
  FOR ALL
  TO authenticated
  USING (user_id = auth.uid());

-- ❌ BAD: Function called per row
CREATE POLICY orders_policy ON orders
  USING (auth.uid() = user_id);  -- Called 1M times for 1M rows!

-- ✅ GOOD: Wrap in SELECT (cached, called once)
CREATE POLICY orders_policy ON orders
  USING ((SELECT auth.uid()) = user_id);  -- 100x faster

-- Always index RLS policy columns
CREATE INDEX orders_user_id_idx ON orders (user_id);

-- ❌ BAD: Overly permissive
GRANT ALL PRIVILEGES ON ALL TABLES TO app_user;

-- ✅ GOOD: Minimal permissions
CREATE ROLE app_readonly NOLOGIN;
GRANT USAGE ON SCHEMA public TO app_readonly;
GRANT SELECT ON public.products, public.categories TO app_readonly;

CREATE ROLE app_writer NOLOGIN;
GRANT USAGE ON SCHEMA public TO app_writer;
GRANT SELECT, INSERT, UPDATE ON public.orders TO app_writer;
-- No DELETE permission

REVOKE ALL ON SCHEMA public FROM public;

-- 4GB RAM example
ALTER SYSTEM SET max_connections = 100;
ALTER SYSTEM SET work_mem = '8MB';  -- 8MB * 100 = 800MB max
SELECT pg_reload_conf();

-- Monitor connections
SELECT count(*), state FROM pg_stat_activity GROUP BY state;

ALTER SYSTEM SET idle_in_transaction_session_timeout = '30s';
ALTER SYSTEM SET idle_session_timeout = '10min';
SELECT pg_reload_conf();

-- ❌ BAD: Lock held during external API call
BEGIN;
SELECT * FROM orders WHERE id = 1 FOR UPDATE;
-- HTTP call takes 5 seconds...
UPDATE orders SET status = 'paid' WHERE id = 1;
COMMIT;

-- ✅ GOOD: Minimal lock duration
-- Do API call first, OUTSIDE transaction
BEGIN;
UPDATE orders SET status = 'paid', payment_id = $1
WHERE id = $2 AND status = 'pending'
RETURNING *;
COMMIT;  -- Lock held for milliseconds

-- ❌ BAD: Inconsistent lock order causes deadlock
-- Transaction A: locks row 1, then row 2
-- Transaction B: locks row 2, then row 1
-- DEADLOCK!

-- ✅ GOOD: Consistent lock order
BEGIN;
SELECT * FROM accounts WHERE id IN (1, 2) ORDER BY id FOR UPDATE;
-- Now both rows locked, update in any order
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT;

-- ❌ BAD: Workers wait for each other
SELECT * FROM jobs WHERE status = 'pending' LIMIT 1 FOR UPDATE;

-- ✅ GOOD: Workers skip locked rows
UPDATE jobs
SET status = 'processing', worker_id = $1, started_at = now()
WHERE id = (
  SELECT id FROM jobs
  WHERE status = 'pending'
  ORDER BY created_at
  LIMIT 1
  FOR UPDATE SKIP LOCKED
)
RETURNING *;

-- ❌ BAD: Individual inserts
INSERT INTO events (user_id, action) VALUES (1, 'click');
INSERT INTO events (user_id, action) VALUES (2, 'view');
-- 1000 round trips

-- ✅ GOOD: Batch insert
INSERT INTO events (user_id, action) VALUES
  (1, 'click'),
  (2, 'view'),
  (3, 'click');
-- 1 round trip

-- ✅ BEST: COPY for large datasets
COPY events (user_id, action) FROM '/path/to/data.csv' WITH (FORMAT csv);

-- ❌ BAD: N+1 pattern
SELECT id FROM users WHERE active = true;  -- Returns 100 IDs
-- Then 100 queries:
SELECT * FROM orders WHERE user_id = 1;
SELECT * FROM orders WHERE user_id = 2;
-- ... 98 more

-- ✅ GOOD: Single query with ANY
SELECT * FROM orders WHERE user_id = ANY(ARRAY[1, 2, 3, ...]);

-- ✅ GOOD: JOIN
SELECT u.id, u.name, o.*
FROM users u
LEFT JOIN orders o ON o.user_id = u.id
WHERE u.active = true;

-- ❌ BAD: OFFSET gets slower with depth
SELECT * FROM products ORDER BY id LIMIT 20 OFFSET 199980;
-- Scans 200,000 rows!

-- ✅ GOOD: Cursor-based (always fast)
SELECT * FROM products WHERE id > 199980 ORDER BY id LIMIT 20;
-- Uses index, O(1)

-- ❌ BAD: Race condition
SELECT * FROM settings WHERE user_id = 123 AND key = 'theme';
-- Both threads find nothing, both insert, one fails

-- ✅ GOOD: Atomic UPSERT
INSERT INTO settings (user_id, key, value)
VALUES (123, 'theme', 'dark')
ON CONFLICT (user_id, key)
DO UPDATE SET value = EXCLUDED.value, updated_at = now()
RETURNING *;

CREATE EXTENSION IF NOT EXISTS pg_stat_statements;

-- Find slowest queries
SELECT calls, round(mean_exec_time::numeric, 2) as mean_ms, query
FROM pg_stat_statements
ORDER BY mean_exec_time DESC
LIMIT 10;

-- Find most frequent queries
SELECT calls, query
FROM pg_stat_statements
ORDER BY calls DESC
LIMIT 10;

EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT)
SELECT * FROM orders WHERE customer_id = 123;

-- Analyze specific table
ANALYZE orders;

-- Check when last analyzed
SELECT relname, last_analyze, last_autoanalyze
FROM pg_stat_user_tables
ORDER BY last_analyze NULLS FIRST;

-- Tune autovacuum for high-churn tables
ALTER TABLE orders SET (
  autovacuum_vacuum_scale_factor = 0.05,
  autovacuum_analyze_scale_factor = 0.02
);

-- GIN index for containment operators
CREATE INDEX products_attrs_gin ON products USING gin (attributes);
SELECT * FROM products WHERE attributes @> '{"color": "red"}';

-- Expression index for specific keys
CREATE INDEX products_brand_idx ON products ((attributes->>'brand'));
SELECT * FROM products WHERE attributes->>'brand' = 'Nike';

-- jsonb_path_ops: 2-3x smaller, only supports @>
CREATE INDEX idx ON products USING gin (attributes jsonb_path_ops);

-- Add generated tsvector column
ALTER TABLE articles ADD COLUMN search_vector tsvector
  GENERATED ALWAYS AS (
    to_tsvector('english', coalesce(title,'') || ' ' || coalesce(content,''))
  ) STORED;

CREATE INDEX articles_search_idx ON articles USING gin (search_vector);

-- Fast full-text search
SELECT * FROM articles
WHERE search_vector @@ to_tsquery('english', 'postgresql & performance');

-- With ranking
SELECT *, ts_rank(search_vector, query) as rank
FROM articles, to_tsquery('english', 'postgresql') query
WHERE search_vector @@ query
ORDER BY rank DESC;