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MCP + FastAPI + PostgreSQL + Ollama Example

This project demonstrates how to build a Minimal Chat Protocol (MCP) server with FastAPI, integrate it with a PostgreSQL database, and use a local Ollama LLM to generate safe SQL queries that answer natural language questions.

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🚀 Features

• MCP-compatible FastAPI server

• Tool query_db that:

  • Generates SQL queries with Ollama
  • Executes them against PostgreSQL
  • Returns JSON results

• Custom /ask API endpoint + HTML UI for direct queries

• Example schema: users, products, orders, order_items, etc.

• Safe SQL: only SELECT queries are executed

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🚀 Architecture

🔄 Flow

1. User Question → sent from the HTML UI (/ask) to FastAPI.
2. FastAPI + MCP Server → passes the question to Ollama.
3. Ollama LLM → generates a SQL SELECT query (only read-only).
4. SQL Validator → ensures query is safe (no DESCRIBE, SHOW, or modifications).
5. PostgreSQL DB → executes the safe SQL query and returns results.
6. FastAPI → formats results into JSON.
7. Response → returned to the user via the HTML UI.

🛠️ Prerequisites

• Python 3.11+
• PostgreSQL running locally on port 5432
• Ollama installed with a SQL-friendly model (e.g. llama3 or codellama)

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📦 Setup Instructions

1. Clone the repository

git clone https://github.com/nutanlade/mcp-llm-db.git
cd mcp-llm-db

2. Create Python virtual environment

# Ensure Python 3.11 is installed
python3.11 -m venv .venv
source .venv/bin/activate

3. Install dependencies

pip install --upgrade pip
pip install fastapi uvicorn psycopg2-binary sqlalchemy fastmcp ollama

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🗄️ Database Setup

1. Create the database

createdb mcpexample

2. Connect to Postgres

psql -U <your_username> -d mcpexample

3. Create schema

CREATE TABLE users (
    id SERIAL PRIMARY KEY,
    name VARCHAR(100) NOT NULL,
    email VARCHAR(100) UNIQUE NOT NULL,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

CREATE TABLE products (
    id SERIAL PRIMARY KEY,
    name VARCHAR(100) NOT NULL,
    description TEXT,
    price NUMERIC(10,2) NOT NULL,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

CREATE TABLE inventories (
    product_id INT PRIMARY KEY REFERENCES products(id) ON DELETE CASCADE,
    quantity INT NOT NULL DEFAULT 0,
    updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

CREATE TABLE orders (
    id SERIAL PRIMARY KEY,
    user_id INT REFERENCES users(id) ON DELETE CASCADE,
    order_date TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    status VARCHAR(20) DEFAULT 'pending'
);

CREATE TABLE order_items (
    id SERIAL PRIMARY KEY,
    order_id INT REFERENCES orders(id) ON DELETE CASCADE,
    product_id INT REFERENCES products(id) ON DELETE CASCADE,
    quantity INT NOT NULL,
    price NUMERIC(10,2) NOT NULL,
    UNIQUE(order_id, product_id)
);

4. Insert sample data

INSERT INTO users (name, email) VALUES
('Alice', 'alice@example.com'),
('Bob', 'bob@example.com');

INSERT INTO products (name, description, price) VALUES
('Laptop', 'High performance laptop', 1500.00),
('Phone', 'Smartphone with OLED screen', 900.00),
('Headphones', 'Noise cancelling headphones', 250.00);

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🤖 Running Ollama

Pull a model that works well with SQL generation (e.g. llama3):

ollama pull llama3

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🖥️ Running the Server

Start FastAPI app

uvicorn main:app --reload

The server will run at: 👉 http://localhost:8000

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📡 Endpoints

/mcp

• MCP-compatible tool interface
• Exposes query_db as a tool

/ask

• Accepts a question (string) and returns:

  • SQL generated by Ollama
  • Query results from Postgres

Example via curl:

curl -X POST http://localhost:8000/ask \
     -F "question=show me top 5 costly products"

Response:

{
  "ok": true,
  "sql": "SELECT name, price FROM products ORDER BY price DESC LIMIT 5;",
  "rows": [
    {"name": "Laptop", "price": 1500.0},
    {"name": "Phone", "price": 900.0},
    {"name": "Headphones", "price": 250.0}
  ]
}

/

• Simple HTML form UI for manual queries

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📝 Notes

• If you see Decimal not JSON serializable, FastAPI’s jsonable_encoder will handle it.
• If you get role "postgres" does not exist, create the role or update your DB URL.
• For ambiguous queries (“costly products”), improve prompt examples to clarify unit price vs sales revenue.

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✅ Example Questions

• Show me the top 5 costly products by price
• Which products generated the highest sales revenue?
• Who placed the most orders?
• List all users and their total order count

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