A full text index allows for a query to quickly search for all occurrences of individual words within longer text strings. Here's an example of building a full text index of Shakespeare's plays.
CREATE TABLE corpus AS
SELECT * FROM read_parquet(
'https://blobs.duckdb.org/data/shakespeare.parquet');
DESCRIBE corpus;
┌─────────────┬─────────────┬─────────┐
│ column_name │ column_type │ null │
├─────────────┼─────────────┼─────────┤
│ line_id │ VARCHAR │ YES │
│ play_name │ VARCHAR │ YES │
│ line_number │ VARCHAR │ YES │
│ speaker │ VARCHAR │ YES │
│ text_entry │ VARCHAR │ YES │
└─────────────┴─────────────┴─────────┘
The text of each line is in text_entry, and a unique key for each line is in line_id.
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.
INSTALL fts;
LOAD fts;
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;
┌───────────────────┬─────────────┬──────────────────────┬──────────────┬──────────────────────────────────────────────┐
│ score │ line_id │ play_name │ speaker │ text_entry │
│ double │ varchar │ varchar │ varchar │ varchar │
├───────────────────┼─────────────┼──────────────────────┼──────────────┼──────────────────────────────────────────────┤
│ 2.683490686835495 │ H4/2.4.115 │ Henry IV │ PRINCE HENRY │ Didst thou never see Titan kiss a dish of … │
│ 3.781282331450016 │ H4/1.2.21 │ Henry IV │ FALSTAFF │ prologue to an egg and butter. │
│ 3.781282331450016 │ H4/2.1.55 │ Henry IV │ Chamberlain │ They are up already, and call for eggs and… │
│ 3.781282331450016 │ H4/4.2.21 │ Henry IV │ FALSTAFF │ toasts-and-butter, with hearts in their be… │
│ 3.781282331450016 │ H4/4.2.62 │ Henry IV │ PRINCE HENRY │ already made thee butter. But tell me, Jac… │
│ 3.781282331450016 │ AWW/4.1.40 │ Alls well that end… │ PAROLLES │ butter-womans mouth and buy myself another… │
│ 3.781282331450016 │ AWW/5.2.9 │ Alls well that end… │ Clown │ henceforth eat no fish of fortunes butteri… │
│ 3.781282331450016 │ AYLI/3.2.93 │ As you like it │ TOUCHSTONE │ right butter-womens rank to market. │
│ 3.781282331450016 │ KL/2.4.132 │ King Lear │ Fool │ kindness to his horse, buttered his hay. │
│ 3.781282331450016 │ MWW/2.2.260 │ Merry Wives of Win… │ FALSTAFF │ Hang him, mechanical salt-butter rogue! I … │
│ 3.781282331450016 │ MWW/2.2.284 │ Merry Wives of Win… │ FORD │ rather trust a Fleming with my butter, Par… │
│ 3.781282331450016 │ MWW/3.5.7 │ Merry Wives of Win… │ FALSTAFF │ Ill have my brains taen out and buttered, … │
│ 3.781282331450016 │ MWW/3.5.102 │ Merry Wives of Win… │ FALSTAFF │ to heat as butter; a man of continual diss… │
│ 6.399093176300027 │ H4/2.4.494 │ Henry IV │ Carrier │ As fat as butter. │
├───────────────────┴─────────────┴──────────────────────┴──────────────┴──────────────────────────────────────────────┤
│ 14 rows 5 columns │
└──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
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 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.