// Database schema design, optimization, and migration patterns for PostgreSQL, MySQL, and NoSQL databases. Use for designing schemas, writing migrations, or optimizing queries.
| name | database-design |
| description | Database schema design, optimization, and migration patterns for PostgreSQL, MySQL, and NoSQL databases. Use for designing schemas, writing migrations, or optimizing queries. |
| source | wshobson/agents |
| license | MIT |
-- 1NF: Atomic values, no repeating groups
-- 2NF: No partial dependencies on composite keys
-- 3NF: No transitive dependencies
-- Users table (normalized)
CREATE TABLE users (
id SERIAL PRIMARY KEY,
email VARCHAR(255) UNIQUE NOT NULL,
created_at TIMESTAMPTZ DEFAULT NOW()
);
-- Addresses table (separate entity)
CREATE TABLE addresses (
id SERIAL PRIMARY KEY,
user_id INTEGER REFERENCES users(id) ON DELETE CASCADE,
street VARCHAR(255),
city VARCHAR(100),
country VARCHAR(100),
is_primary BOOLEAN DEFAULT false
);
-- When read performance matters more than write consistency
CREATE TABLE order_summaries (
id SERIAL PRIMARY KEY,
order_id INTEGER REFERENCES orders(id),
customer_name VARCHAR(255), -- Denormalized from customers
total_amount DECIMAL(10,2),
item_count INTEGER,
last_updated TIMESTAMPTZ DEFAULT NOW()
);
-- B-tree (default) for equality and range queries
CREATE INDEX idx_users_email ON users(email);
-- Composite index (order matters!)
CREATE INDEX idx_orders_user_date ON orders(user_id, created_at DESC);
-- Partial index for specific conditions
CREATE INDEX idx_active_users ON users(email) WHERE deleted_at IS NULL;
-- GIN index for array/JSONB columns
CREATE INDEX idx_posts_tags ON posts USING GIN(tags);
-- Covering index (includes additional columns)
CREATE INDEX idx_orders_covering ON orders(user_id) INCLUDE (total, status);
-- Check index usage
SELECT
schemaname, tablename, indexname,
idx_scan, idx_tup_read, idx_tup_fetch
FROM pg_stat_user_indexes
ORDER BY idx_scan DESC;
-- Find missing indexes
SELECT
relname, seq_scan, seq_tup_read,
idx_scan, idx_tup_fetch
FROM pg_stat_user_tables
WHERE seq_scan > idx_scan
ORDER BY seq_tup_read DESC;
-- Always use transactions
BEGIN;
-- Add column with default (non-blocking in PG 11+)
ALTER TABLE users ADD COLUMN status VARCHAR(20) DEFAULT 'active';
-- Create index concurrently (doesn't lock table)
CREATE INDEX CONCURRENTLY idx_users_status ON users(status);
-- Backfill data in batches
UPDATE users SET status = 'active' WHERE status IS NULL AND id BETWEEN 1 AND 10000;
COMMIT;
1. Add new column (nullable)
2. Deploy code that writes to both columns
3. Backfill old data
4. Deploy code that reads from new column
5. Remove old column
-- Always use EXPLAIN ANALYZE
EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT)
SELECT * FROM orders WHERE user_id = 123 AND status = 'pending';
-- Key metrics to watch:
-- - Seq Scan vs Index Scan
-- - Actual rows vs Estimated rows
-- - Buffers: shared hit vs read
-- Use EXISTS instead of IN for large sets
SELECT * FROM users u
WHERE EXISTS (SELECT 1 FROM orders o WHERE o.user_id = u.id);
-- Pagination with keyset (cursor) instead of OFFSET
SELECT * FROM posts
WHERE created_at < '2024-01-01'
ORDER BY created_at DESC
LIMIT 20;
-- Use CTEs for complex queries
WITH active_users AS (
SELECT id FROM users WHERE last_login > NOW() - INTERVAL '30 days'
)
SELECT * FROM orders WHERE user_id IN (SELECT id FROM active_users);
-- Primary key
ALTER TABLE users ADD PRIMARY KEY (id);
-- Foreign key with cascade
ALTER TABLE orders ADD CONSTRAINT fk_orders_user
FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE;
-- Check constraint
ALTER TABLE products ADD CONSTRAINT chk_price_positive
CHECK (price >= 0);
-- Unique constraint
ALTER TABLE users ADD CONSTRAINT uniq_users_email UNIQUE (email);
-- Exclusion constraint (no overlapping ranges)
ALTER TABLE reservations ADD CONSTRAINT excl_no_overlap
EXCLUDE USING gist (room_id WITH =, tsrange(start_time, end_time) WITH &&);
created_at and updated_at timestampsdeleted_at) for important data