How Can I Efficiently Find Similar Strings in PostgreSQL?

Finding Similar Strings Efficiently in PostgreSQL

Intro: Finding similar strings in large datasets can encounter performance issues when using conventional methods. This article presents a solution that significantly speeds up the search process by employing PostgreSQL's pg_trgm module.

Using SET pg_trgm.similarity_threshold and the % Operator:

The query you provided suffers from excessive similarity calculations. To enhance efficiency, utilize the SET pg_trgm.similarity_threshold configuration parameter and the % operator:

SET pg_trgm.similarity_threshold = 0.8;

SELECT similarity(n1.name, n2.name) AS sim, n1.name, n2.name
FROM names n1
JOIN names n2 ON n1.name <> n2.name
AND n1.name % n2.name
ORDER BY sim DESC;

This approach leverages a trigram GiST index, significantly accelerating the search.

Utilizing Functional Indexes:

To further improve performance, consider employing functional indexes to prefilter possible matches before the cross join. This reduces the number of similarity calculations required, as demonstrated in the following query:

CREATE FUNCTION first_char(text) RETURNS text AS $$
  SELECT substring(, 1, 1);
$$ LANGUAGE SQL;

CREATE INDEX first_char_idx ON names (first_char(name));

SELECT similarity(n1.name, n2.name) AS sim, n1.name, n2.name
FROM names n1
JOIN names n2 ON first_char(n1.name) = first_char(n2.name)
AND n1.name <> n2.name
ORDER BY sim DESC;

Conclusion:

By employing the pg_trgm module, SET pg_trgm.similarity_threshold, the % operator, and functional indexes, you can dramatically enhance the performance of finding similar strings in PostgreSQL, even for large datasets.

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