# Full-Text Search with BM25 Ranking

Use `lakebase_text` for BM25 relevance ranking with PostgreSQL's standard `tsvector` type. The `lakebase_bm25` index adds corpus-aware ranking and top-K pushdown.

Lakebase Search requires Postgres 16 or later. Enable the extension before creating the index:

```sql
CREATE EXTENSION IF NOT EXISTS lakebase_text;
```

`lakebase_text` has no extension dependency. It relies on a preloaded library that Neon enables by default; if the project customized its preloaded-library list, confirm the library remains enabled.

## Prepare and Index Text

Prefer a stored generated `tsvector` when search text comes from stable table columns:

```sql
CREATE TABLE documents (
  id bigint GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
  title text NOT NULL,
  body text NOT NULL,
  body_tsv tsvector GENERATED ALWAYS AS
    (to_tsvector('english', body)) STORED
);
```

Create the index after the initial corpus has been inserted so build-time corpus statistics are meaningful. BM25 scoring is tuned by two storage parameters set at index-build time:

- `k1` controls term-frequency saturation (default `1.2`, range `1.2`–`2.0`): higher values let repeated terms keep adding relevance.

- `b` controls document-length normalization (default `0.75`, range `0.0`–`1.0`): higher values penalize longer documents more.

Both can only be set in the `WITH` clause, and updating them rebuilds the index:

```sql
CREATE INDEX documents_body_bm25 ON documents
  USING lakebase_bm25 (body_tsv)
  WITH (k1 = 1.2, b = 0.75);
```

After a large bulk load, run `VACUUM` to refresh the statistics used by BM25 scoring.

## Query and Interpret Scores

`to_bm25query` binds the query `tsvector` to the BM25 index whose corpus statistics should be used. The `<@>` operator returns a negative BM25 score, so lower (more negative) values are more relevant and must sort ascending:

```sql
SELECT
  id,
  title,
  body_tsv <@> to_bm25query(
    to_tsvector('english', $1),
    'documents_body_bm25'::regclass
  ) AS score
FROM documents
ORDER BY score
LIMIT $2;
```

Use the same text-search configuration for document and query vectors. Select a language-specific or custom configuration that matches the corpus.

## Set the Candidate Limit

`lakebase_bm25.default_limit` controls how many rows the index returns before PostgreSQL applies the SQL `LIMIT`. Its default is `1000`; setting it close to the requested top-K avoids unnecessary scoring.

## Use Prefilter Selectively

Enable prefilter when a `WHERE` condition is strict or unpredictable and cheap to evaluate. It lets the index prune rows before BM25 scoring. A loose or expensive filter can be slower with prefilter enabled.

```sql
BEGIN;
SET LOCAL lakebase_bm25.default_limit = 20;
SET LOCAL lakebase_bm25.prefilter = on;

SELECT
  id,
  title,
  body_tsv <@> to_bm25query(
    to_tsvector('english', $1),
    'documents_body_bm25'::regclass
  ) AS score
FROM documents
WHERE id % 1000 = 0
ORDER BY score
LIMIT $2;
COMMIT;
```

## Set Parameters at Build Time or Per Query

Several BM25 parameters can be set in two places. As an index storage parameter in the `CREATE INDEX` `WITH` clause, a value is fed into the index as its build-time default. As a session GUC via `SET` (or `SET LOCAL`), it applies per query and takes precedence over the stored default when both are present.

- `default_limit` and `prefilter` exist in both forms: set an index default that fits the common case, then override it per query with a GUC without rebuilding.
- `k1` (default `1.2`) and `b` (default `0.75`) are storage parameters only. There is no GUC for them.
- `enable_scan` (default `on`) is a GUC only.

The examples use `SET LOCAL` so each override is scoped to its own transaction, which is required behind a connection pool or stateless driver.

Source: [`lakebase_text` documentation](https://neon.com/docs/extensions/lakebase-text).