Understanding Vector Similarity Search with FAISS: A Deep Dive

Posted by Ibrahim Cikrikcioglu on October 13, 2023 · 6 mins read

Ever wandered into the world of vector embeddings and pondered how to swiftly search through vast amounts of data? Enter FAISS: a robust solution by Facebook AI Research. Let’s dive deep into this technology.

1. Introduction to FAISS

Developed by the tech giant’s very own Facebook AI Research (FAIR) team, FAISS stands tall as a robust library specifically designed for similarity search and clustering of dense vectors on a large scale. What makes it a go-to for many is its optimization for in-memory operations. This is a boon for applications demanding real-time results, such as open-domain question-answering systems.

2. Role of Vector Embeddings

But before we venture any further, it’s crucial to understand the underlying concept: vector embeddings. In essence, this method transforms various forms of data – may it be text, images, or more – into vectors that hold the essence of the original data, namely its semantic or contextual information. The primary application? Using these vectors for similarity searches to pinpoint the most relevant items in a dataset.

3. FAISS Indexes: Beyond Simple Storage

However, FAISS isn’t merely a storage system for these vectors. It goes a step further by constructing specialized indexes that are designed and optimized for similarity search. Depending on the nature of your data and your preferences between speed and accuracy, you can choose from different types of indexes.

4. Dynamism in Data: Adding New Conversations

Let’s address the elephant in the room: data is dynamic. As you gather more data, you’d want to add it to your index. While FAISS does support direct additions for some index types, this might not be the most efficient way for larger datasets. In such scenarios, periodically rebuilding your index ensures it remains in top shape. But there’s more: for truly dynamic datasets, sharding is your best friend. This technique involves dividing your data into multiple smaller indexes, making it easy to add new data without overwhelming the system.

5. Memory Considerations: In-memory vs. Disk

Yes, FAISS primarily operates in-memory to deliver the best performance. But what if your dataset is larger than your available RAM? Not to worry! FAISS has provisions for serialization and deserialization, giving you the flexibility to save and load indexes from the disk. Some specialized on-disk indexes like IndexFlat with IDMap2 and IndexIVF with OnDiskInvertedLists are tailored for such situations, though there’s a slight compromise on speed.

6. FAISS and Data Retrieval

One thing to note: FAISS is all about vector representations. It doesn’t store the original data items. Instead, every vector can have a unique ID, and upon searching, FAISS hands you the IDs of the closest vectors. Think of these IDs as your golden ticket or “pointers” to fetch the original data from its storage spot, be it databases or file systems.

7. Strategies for Handling Large and Evolving Datasets

Large datasets bring their own set of challenges. Techniques like sharding can come in handy when your dataset swells beyond the memory’s hold. Additionally, FAISS’s IVF indexes do support the addition of new vectors after their initial training, but remember, there’s a saturation point beyond which performance might dip. For datasets that continually evolve, consider retraining and rebuilding your index periodically.

8. FAISS and Language Models: A Powerful Synergy

The rise of state-of-the-art Language Models (LLMs) like GPT, BERT, and their variants has transformed a myriad of applications from chatbots to content creation tools. These models are adept at generating rich vector embeddings for text, which can then be used for various tasks, including similarity search.

How FAISS comes into play:

Embedding Retrieval for Large Text Corpses: When LLMs are used to generate embeddings for large text datasets, FAISS can be employed to swiftly retrieve the most semantically relevant pieces of text based on a query. This is particularly beneficial for applications like semantic search engines and content recommendation systems.

Training Data Selection for Fine-Tuning LLMs: When fine-tuning LLMs for specific tasks, one might need to select relevant data from a vast corpus. Using the embeddings produced by the base LLM, FAISS can help identify and retrieve the most pertinent data samples for fine-tuning.

Real-time Applications: For products that require real-time responses, such as chatbots powered by LLMs, FAISS ensures that the bot can quickly search through its knowledge base (represented as embeddings) to fetch or generate the most appropriate response.

Integrating with Language-Model Based Products:

Scalability: Language model-based products, especially those dealing with vast textual data, benefit from FAISS’s ability to handle large-scale similarity searches efficiently.

Data Enrichment: As LLMs continually generate new content or process user interactions, FAISS indexes can be updated or rebuilt periodically to incorporate this new data, ensuring the search remains relevant and up-to-date.

9. Combining FAISS with Traditional Databases

To get the best of both worlds, one can harmoniously integrate FAISS with traditional databases. This combination results in a powerful system where FAISS takes charge of vector similarity search, and databases handle the storage, retrieval, and management of the actual data.


In summary, as the digital era continually produces vast amounts of data, tools like FAISS pave the way for efficient, real-time data retrieval, making the task of searching through dense vectors not just feasible but incredibly efficient.