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TriLite, An Inverted Trigram Index for Accelerated String Matching in Sqlite

TriLite is an experimental Sqlite extension for creating an inverted trigram index to accelerate string matching queries, such as regular expressions and substring matching.

A trigram is essential any substring of length 3, the inverted index maps from trigrams, to a list of document ids for documents containing the given trigram. When querying for a substring, trigrams is extracted from the substring, and documents id lists for the trigrams is fetched and merged, after which only documents containing all the trigrams are matched against the substring and returned if considered a result.

The approach is pretty much How Google Code Search Worked, and similar to the pg_trgm module for PostgreSQL. See related work for more information.


TriLite sources is unless otherwise noted released into public domain.

Current State

TriLite is under active development and currently supports the bare minimum to make it usable for static databases, ie. INSERT and accelerated substring and regular expression matching.

TriLite still has a lot of short comings, and this code is a work in progress. To sum this up do NOT use TriLite in a production environment, unless you are me. Currently TriLite can do a few tricks as outlined in test.sql, which you can pipe to sqlite3 after building TriLite.

Things To Do

  • Add reference counted regular expression -> compiled expression cache to vtable
  • Add rowid, column -> first match cache to regular expression cache entries
  • Support multiple columns of different types
    • Alias rowid, with PRIMARY INTEGER KEY
    • Auxiliary non-indexed columns (useful for storing extents and file id)
    • Indexed text columns (useful for tname and tqualname)
  • Handle extents for expressions with the empty string in the language
  • Limit number of extents returned per file by queries
  • Facilitate update/delete
  • Use a hash set to optimize extraction of unique trigrams
  • Optimize temporary hash table
  • Query documents inserted in the same transaction, ie. before commit as currently required
  • Table specific options, configurable at runtime not compile time
    • Forbid full table scan with MATCH using regular expressions
    • Max pending insert cache (bytes)
    • Max pending insert list
    • Max pending delete cache (bytes)
    • Max pending delete list
  • Handle LIKE/GLOB patterns (ie. case (in)sensitive ordered substrings)
  • Write Documentation


TriLite would not been as fast without these awesome projects.

  • re2 fast regular expression engine.
  • Railgun_Doublet fast strstr-like substring matching.

Related Work

Other projects with similar aim,


An inverted trigram index for accelerated string matching in Sqlite.






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