Embeddings

Embeddings are a numeric representation of text. They are used to represent words and sentences as vectors, an array of numbers. Embeddings can be used to find similar pieces of text, by comparing the similarity of the numeric vectors using a distance measure, or they can be used as input features for other machine learning models, since most algorithms can't use text directly.

Many pretrained LLMs can be used to generate embeddings from text within PostgresML. You can browse all the models available to find the best solution on Hugging Face.

PostgresML provides a simple interface to generate embeddings from text in your database. You can use the pgml.embed function to generate embeddings for a column of text. The function takes a transformer name and a text value. The transformer will automatically be downloaded and cached for reuse.

Long Form Examples

For a deeper dive, check out the following articles we've written illustrating the use of embeddings:

API

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pgml.embed(
transformer TEXT, -- huggingface sentence-transformer name
text TEXT, -- input to embed
kwargs JSON -- optional arguments (see below)
)

Example

Let's use the pgml.embed function to generate embeddings for tweets, so we can find similar ones. We will use the distilbert-base-uncased model. This model is a small version of the bert-base-uncased model. It is a good choice for short texts like tweets. To start, we'll load a dataset that provides tweets classified into different topics.

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SELECT pgml.load_dataset('tweet_eval', 'sentiment');

View some tweets and their topics.

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SELECT *
FROM pgml.tweet_eval
LIMIT 10;

Get a preview of the embeddings for the first 10 tweets. This will also download the model and cache it for reuse, since it's the first time we've used it.

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SELECT text, pgml.embed('distilbert-base-uncased', text)
FROM pgml.tweet_eval
LIMIT 10;

It will take a few minutes to generate the embeddings for the entire dataset. We'll save the results to a new table.

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CREATE TABLE tweet_embeddings AS
SELECT text, pgml.embed('distilbert-base-uncased', text) AS embedding
FROM pgml.tweet_eval;

Now we can use the embeddings to find similar tweets. We'll use the pgml.cosign_similarity function to find the tweets that are most similar to a given tweet (or any other text input).

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WITH query AS (
SELECT pgml.embed('distilbert-base-uncased', 'Star Wars christmas special is on Disney') AS embedding
)
SELECT text, pgml.cosine_similarity(tweet_embeddings.embedding, query.embedding) AS similarity
FROM tweet_embeddings, query
ORDER BY similarity DESC
LIMIT 50;
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WITH query AS (
SELECT pgml.embed('sentence-transformers/all-MiniLM-L6-v2', 'Star Wars christmas special is on Disney') AS embedding
)
SELECT text, pgml.cosine_similarity(tweet_embeddings_2.embedding, query.embedding) AS similarity
FROM tweet_embeddings_2, query
ORDER BY similarity DESC
LIMIT 50;

On small datasets (<100k rows), a linear search that compares every row to the query will give sub-second results, which may be fast enough for your use case. For larger datasets, you may want to consider various indexing strategies offered by additional extensions.

  • Cube is a built-in extension that provides a fast indexing strategy for finding similar vectors. By default it has an arbitrary limit of 100 dimensions, unless Postgres is compiled with a larger size.
  • PgVector supports embeddings up to 2000 dimensions out of the box, and provides a fast indexing strategy for finding similar vectors.
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CREATE EXTENSION vector;
CREATE TABLE items (text text, embedding vector(384));
insert into items select text, embedding from tweet_embeddings_2;
WITH query AS (
SELECT pgml.embed('sentence-transformers/all-MiniLM-L6-v2', 'Star Wars christmas special is on Disney')::vector AS embedding
)
SELECT * FROM items, query ORDER BY items.embedding <-> query.embedding LIMIT 10;
CREATE INDEX ON tweet_embeddings_2 USING ivfflat (embedding vector_cosine_ops);

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