Similarity Search in PostgreSQL

PostgreSQL provides the similarity() function to calculate the similarity between two strings. This function can be used to find similar data in the same table.

For example, let's say we have a table that stores information about web pages, with a column called title that represents the web page's title. We can use the following SQL query to find web pages with titles that are more than 0.9 similar:

SELECT a.id, b.id, similarity(a.title, b.title) as similarity FROM page AS a
JOIN page as b ON similarity(a.title, b.title) > 0.9
WHERE a.id <> b.id ORDER BY similarity desc;

This query will return all rows from the two tables page, where a.id and b.id represent two different web pages. The similarity(a.title, b.title) function will return the similarity between the two strings a.title and b.title.

The return value of the similarity() function is a floating-point number between 0 and 1. A higher value indicates that the two strings are more similar.

In the above query, we use the JOIN statement to join the two tables page. This is to ensure that the results set contains two different web pages for a.id and b.id.

1. How to Enable Similarity Search in PostgreSQL

To enable similarity search in PostgreSQL, you need to install the pgtrgm extension. This extension provides a number of functions for calculating the similarity between strings using trigram matching.

To install the pg_trgm extension, run the following command:

CREATE EXTENSION pg_trgm;

Once the pg_trgm extension is installed, you can use the similarity() function to calculate the similarity between two strings.

In conclusion, the PostgreSQL similarity() function provides an efficient way to find similar data in the same table.