DuckDB Full Text Search
Version 0.8.1

A full text index allows for a query to quickly search for all occurences 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(
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,
  FROM corpus
  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

Details are here

  • 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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