Why File When You Can Full-Text Search
Feb 19, 2008

Why file (or, in databases, categorize) when you can search? Although full-text search doesn't preclude other useful devices such as tagging and constraining searches by SQL columns, for the most part, full-text search is as close to a perfect information retrieval device as the memex could be.

Here is how to accomplish full-text search using SQLite and Perl DBD::SQLite.

  1. Install Perl DBD::SQLite
    1. If you already have SQLite installed on your computer, move or rename /usr/local/lib/libsqlite* to something else so the new DBD::SQLite install is forced to use its own source.
    2. Download Audrey Tang's most excellent DBD::SQLite::Amalgamation-3.5.6 from CPAN. Yes, we will use the SQLite Amalgamation instead of the source code bits-and-bobs. Audrey's version magically does all the work and then disappears -- you still use DBD::SQLite via DBI, all the same syntax and all, but you get the latest source code all in one slurpee.
    3. un-gzip-tar the source code for the Perl module.
    4. Edit Makefile.PL. Add the flag -DSQLITE_ENABLE_FTS3=1 to the 'DEFINE' key. It is line 135 in my Makefile.PL. I just added it right at the beginning. So, my 'DEFINE' key now looks like
      'DEFINE' => "-DSQLITE_ENABLE_FTS3=1 -DSQLITE_CORE -DNDEBUG=1 -DSQLITE_PTR_SZ=$Config{ptrsize}"
    5. make && sudo make install
  2. Install the latest SQLite with full-text search (for command line operation)
    1. Download the latest SQLite amalgamation from the mothership.
    2. run the following command in the src directory
      CFLAGS="-Os -DSQLITE_ENABLE_FTS3=1" ./configure
    3. Finally, make && sudo make install

Alright. Now everything is installed. What to do now?

Let's imagine you have a bunch of papers (text files) that you want to load into a database and enable full-text search on it. Your schema is

CREATE TABLE paper (
    paper_id INTEGER PRIMARY KEY, 
    paper_name TEXT, 
    paper_text TEXT
);

Load your papers into the above table. It will automatically get its primary key populated by SQLite.

Now, create the following virtual table

CREATE VIRTUAL TABLE fts_paper 
USING fts3 (paper_name, paper_text);

Now, load data into it from your main table paper.

INSERT INTO fts_paper (rowid, paper_name, paper_text) 
SELECT paper_id, paper_name, paper_text FROM paper

Fts will do its magic and create a few of its own tables. In my world they look like

CREATE TABLE fts_paper_content (
  docid INTEGER PRIMARY KEY,
  c0paper_name, 
  c1paper_text
);

CREATE TABLE fts_paper_segdir (
  level INTEGER,  
  idx INTEGER,  
  start_block INTEGER,  
  leaves_end_block INTEGER,  
  end_block INTEGER,  
  root BLOB,  
  PRIMARY KEY(level, idx)
);

CREATE TABLE fts_paper_segments (  
  blockid INTEGER PRIMARY KEY,  
  block BLOB
);

But, leave those tables alone. But, you can now search your papers using

SELECT a.paper_id, a.paper_name, snippet(b.paper_text) 
FROM paper a JOIN fts_paper b ON a.paper_id = b.rowid 
WHERE b.paper_text MATCH 'automatically';

Assuming this wiki page has been inserted into the full-text index, you will get back (the following results are broken up on multiple lines)

543 | 
Why File When You Can Full-Text Search | 
... papers into the above table. It will 
automatically get its primary key populated by 
SQLite. ...

Final step. Create a few TRIGGERs to automatically update the fts index on INSERTs and UPDATEs

CREATE TRIGGER update_fts 
AFTER UPDATE OF paper_text ON paper 
BEGIN
  UPDATE fts_paper 
  SET paper_text = new.paper_text 
  WHERE rowid = old.paper_id;
END

CREATE TRIGGER insert_fts 
AFTER INSERT ON paper 
BEGIN
  INSERT INTO fts_paper (rowid, paper_text) 
  VALUES (new.paper_id, new.paper_text);
END

Enjoy.

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