Contribution to Hugging Face 🤗 cookbook: Add a Lang Chain agent that can interact with a PostgreSQL database

[paartheee]

We can use the PostgreSQL database to feed additional context to LLMs. However, the naive approach to generating database queries is currently too brittle. Therefore, it is advisable to implement an agent and give it tools to interact with a graph database. The tools use predefined flows and allow agents to solve specific problems. we can use Mixtral 8x7b as an agent.