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Full-Text Search

DuckDB supports full-text search via the fts extension. A full-text index allows for a query to quickly search for all occurrences of individual words within longer text strings.

Example: Shakespeare Corpus

Here's an example of building a full-text index of Shakespeare's plays.

CREATE TABLE corpus AS
    SELECT * FROM 'https://blobs.duckdb.org/data/shakespeare.parquet';

DESCRIBE corpus;
column_name column_type null key default extra
line_id VARCHAR YES NULL NULL NULL
play_name VARCHAR YES NULL NULL NULL
line_number VARCHAR YES NULL NULL NULL
speaker VARCHAR YES NULL NULL NULL
text_entry VARCHAR YES NULL NULL NULL

The text of each line is in text_entry, and a unique key for each line is in line_id.

Creating a Full-Text Search Index

First, we create the index, specifying the table name, the unique id column, and the column(s) to index. We will just index the single column text_entry, which contains the text of the lines in the play.

PRAGMA create_fts_index('corpus', 'line_id', 'text_entry');

The table is now ready to query using the Okapi BM25 ranking function. Rows with no match return a null score.

What does Shakespeare say about butter?

SELECT
    fts_main_corpus.match_bm25(line_id, 'butter') AS score,
    line_id, play_name, speaker, text_entry
FROM corpus
WHERE score IS NOT NULL
ORDER BY score DESC;
score line_id play_name speaker text_entry
4.427313429798464 H4/2.4.494 Henry IV Carrier As fat as butter.
3.836270302568675 H4/1.2.21 Henry IV FALSTAFF prologue to an egg and butter.
3.836270302568675 H4/2.1.55 Henry IV Chamberlain They are up already, and call for eggs and butter;
3.3844488405497115 H4/4.2.21 Henry IV FALSTAFF toasts-and-butter, with hearts in their bellies no
3.3844488405497115 H4/4.2.62 Henry IV PRINCE HENRY already made thee butter. But tell me, Jack, whose
3.3844488405497115 AWW/4.1.40 Alls well that ends well PAROLLES butter-womans mouth and buy myself another of
3.3844488405497115 AYLI/3.2.93 As you like it TOUCHSTONE right butter-womens rank to market.
3.3844488405497115 KL/2.4.132 King Lear Fool kindness to his horse, buttered his hay.
3.0278411214953107 AWW/5.2.9 Alls well that ends well Clown henceforth eat no fish of fortunes buttering.
3.0278411214953107 MWW/2.2.260 Merry Wives of Windsor FALSTAFF Hang him, mechanical salt-butter rogue! I will
3.0278411214953107 MWW/2.2.284 Merry Wives of Windsor FORD rather trust a Fleming with my butter, Parson Hugh
3.0278411214953107 MWW/3.5.7 Merry Wives of Windsor FALSTAFF Ill have my brains taen out and buttered, and give
3.0278411214953107 MWW/3.5.102 Merry Wives of Windsor FALSTAFF to heat as butter; a man of continual dissolution
2.739219044070792 H4/2.4.115 Henry IV PRINCE HENRY Didst thou never see Titan kiss a dish of butter?

Unlike standard indexes, full-text indexes don't auto-update as the underlying data is changed, so you need to PRAGMA drop_fts_index(my_fts_index) and recreate it when appropriate.

Note on Generating the Corpus Table

For more details, see the “Generating a Shakespeare corpus for full-text searching from JSON” blog post

  • The Columns are: line_id, play_name, line_number, speaker, text_entry.
  • We need a unique key for each row in order for full-text searching to work.
  • The line_id KL/2.4.132 means King Lear, Act 2, Scene 4, Line 132.
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