/database-patterns

name: database-patterns
description: "DB schema design and query tuning: normalization, indexing, N+1, transactions, EXPLAIN. Triggers: schema, index, slow query, N+1, PostgreSQL, MySQL, EXPLAIN, deadlock, query plan."
effort: medium
user-invocable: false
allowed-tools: Read

Database Patterns Skill

ORM Selection

| Scenario | ORM | |----------|-----| | Node.js, type-safe | Prisma | | Node.js, SQL-first | Drizzle | | Python, async | SQLAlchemy 2.0 | | Python, simple | SQLModel | | PHP | Doctrine, Eloquent |

---

Schema Design

Naming Conventions

-- Tables: plural, snake_case
CREATE TABLE user_profiles (...);

-- Columns: snake_case
user_id, created_at, is_active

-- Indexes: idx_{table}_{columns}
CREATE INDEX idx_users_email ON users(email);

-- Foreign keys: fk_{table}_{ref_table}
CONSTRAINT fk_orders_users FOREIGN KEY (user_id) REFERENCES users(id)

Common Patterns

Soft Delete

ALTER TABLE users ADD COLUMN deleted_at TIMESTAMP NULL;

-- Query active records
SELECT * FROM users WHERE deleted_at IS NULL;

Audit Columns

created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
created_by UUID REFERENCES users(id),
updated_by UUID REFERENCES users(id)

UUID vs Serial

| Use Case | Type | |----------|------| | Internal only | SERIAL/BIGSERIAL | | External/distributed | UUID | | Human readable | SERIAL with prefix |

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Index Strategies

When to Index

• Foreign keys (always)
• Columns in WHERE clauses
• Columns in ORDER BY
• Columns in JOIN conditions

Index Types

| Type | Use Case | |------|----------| | B-tree | Equality, range (default) | | Hash | Equality only | | GIN | Arrays, JSONB, full-text | | GiST | Geometric, full-text | | BRIN | Large sequential data |

Composite Index Order

-- Good: matches query pattern
CREATE INDEX idx_orders_user_date ON orders(user_id, created_at);
SELECT * FROM orders WHERE user_id = 1 AND created_at > '2024-01-01';

-- Index used for:
-- WHERE user_id = 1
-- WHERE user_id = 1 AND created_at > ...

-- Index NOT used for:
-- WHERE created_at > '2024-01-01' (missing leading column)

---

Query Optimization

Explain Analyze

EXPLAIN ANALYZE
SELECT * FROM users WHERE email = 'test@example.com';

Common Issues

| Issue | Solution | |-------|----------| | Seq Scan on large table | Add index | | High row estimate | Update statistics | | Nested Loop on large sets | Consider hash join | | Sort in memory | Increase work_mem |

N+1 Prevention

# Bad: N+1
for user in users:
    print(user.orders)  # Query per user

# Good: Eager loading
users = User.query.options(joinedload(User.orders)).all()

---

Migration Best Practices

Safe Migrations

-- Add column (safe)
ALTER TABLE users ADD COLUMN phone VARCHAR(20);

-- Add NOT NULL column (safe pattern)
ALTER TABLE users ADD COLUMN phone VARCHAR(20);
UPDATE users SET phone = '' WHERE phone IS NULL;
ALTER TABLE users ALTER COLUMN phone SET NOT NULL;

-- Rename column (use application-level)
-- 1. Add new column
-- 2. Copy data
-- 3. Update application
-- 4. Remove old column

Migration Checklist

• [ ] Tested on production-like data
• [ ] Rollback script ready
• [ ] No long locks on large tables
• [ ] Indexes created concurrently
• [ ] Application handles both states

---

Connection Pooling

PgBouncer Settings

[pgbouncer]
pool_mode = transaction
max_client_conn = 1000
default_pool_size = 20

Application Settings

| Framework | Pool Size Formula | |-----------|------------------| | General | (cores * 2) + disk spindles | | Read-heavy | cores * 4 | | Write-heavy | cores * 2 |

---

Vector Database (Qdrant) Patterns

Client Setup

from qdrant_client import QdrantClient
from qdrant_client.models import Distance, VectorParams, PointStruct

# Sync client
client = QdrantClient(host="localhost", port=6333)

# Async client
from qdrant_client import AsyncQdrantClient
async_client = AsyncQdrantClient(host="localhost", port=6333)

Collection Management

# Create collection (single vector)
client.create_collection(
    collection_name="documents",
    vectors_config=VectorParams(size=384, distance=Distance.COSINE)
)

# Create collection (multi-vector)
from qdrant_client.models import VectorParams

client.create_collection(
    collection_name="multimodal",
    vectors_config={
        "text": VectorParams(size=384, distance=Distance.COSINE),
        "image": VectorParams(size=512, distance=Distance.EUCLID),
    }
)

Upserting Vectors

# Single upsert
client.upsert(
    collection_name="documents",
    points=[
        PointStruct(
            id=1,
            vector=[0.1, 0.2, 0.3, ...],  # 384-dim vector
            payload={"title": "Doc 1", "category": "tech"}
        )
    ]
)

# Batch upsert
points = [
    PointStruct(id=i, vector=vectors[i], payload=payloads[i])
    for i in range(len(vectors))
]
client.upsert(collection_name="documents", points=points, batch_size=100)

Searching Vectors

from qdrant_client.models import Filter, FieldCondition, MatchValue

# Basic search
results = client.search(
    collection_name="documents",
    query_vector=[0.1, 0.2, ...],
    limit=10
)

# Search with filter
results = client.search(
    collection_name="documents",
    query_vector=[0.1, 0.2, ...],
    query_filter=Filter(
        must=[
            FieldCondition(key="category", match=MatchValue(value="tech"))
        ]
    ),
    limit=10,
    with_payload=True,
    score_threshold=0.7
)

# Search with range filter
from qdrant_client.models import Range

results = client.search(
    collection_name="documents",
    query_vector=query_vector,
    query_filter=Filter(
        must=[
            FieldCondition(key="price", range=Range(gte=10, lte=100))
        ]
    ),
    limit=10
)

Payload Indexing

# Create payloa