In this post, I am sharing small demonstration on, how to find similarity between Postgres strings in percentage?
PostgreSQL is a well known for a variety of string functions which are used for data analysis.

One of our developers is generating random token string manually for two columns, and now he is required to find similarity between this string.

In the PostgreSQL, you can use a pg_trgm module to find similarity based on trigram matching.

Below is a demonstration of this:

Create a table with sample data:

CREATE TABLE tbl_SimilarString
(
	Str1 TEXT
	,Str2 TEXT
);

INSERT INTO tbl_SimilarString VALUES
('Anvesh Patel','Anvesh Pat')
,('dbrnd','dbrnd blog')
,('database dev','database developer')
,('postgres database','database postgres');

Install pg_trgm module:

CREATE EXTENSION pg_trgm;

Use similarity():

SELECT similarity(Str1,Str2) FROM tbl_SimilarString;

The result:

similarity
------------
   0.714286
   0.545455
   0.578947
          1
(4 rows)

In the PostgreSQL, we can use Trigram to enhance the use of Full Text Search.
The Trigram algorithm divides string like “dog” = “d”,”do”,”dog”,”og”.

The Trigram matching comes with pg_trgm extension.
We can also apply Trigram operators on Gist and GIN indexes which increase the search speed and enhance the performance.

We have already different thesaurus or synomyms for the Full Text Search, but we can use Trigram for implementing typing corrections and suggestions.

Here, you can find different examples of this.

Create a sample table:

CREATE TABLE tbl_TestTriGramIndex
(
	ID INT
	,TextData TEXT
);

Create a Trigram Index on Text Column:

CREATE INDEX trgm_idx_tbl_TestTriGramIndex_TextData ON tbl_TestTriGramIndex USING GIST (TextData gist_trgm_ops);

If you get below error, install require pg_trgm extension:

ERROR:  operator class "gist_trgm_ops" does not exist for access method "gist"

Install pg_trgm extension:

CREATE EXTENSION pg_trgm;

Insert few sample records:

INSERT INTO tbl_TestTriGramIndex
VALUES 
(1,'Anvesh Patel')
,(2,'Database Research & Development')
,(3,'PostgreSQL is better than SQL Server')
,(4,'PostgreSQL is supported powerful BRIN Indexes');

Execute few SELECTs for text comparison:

SELECT *FROM tbl_TestTriGramIndex
WHERE TextData % 'PostgreSQL is';

SELECT *FROM tbl_TestTriGramIndex
WHERE TextData LIKE '%SQL%';

SELECT *FROM tbl_TestTriGramIndex
WHERE TextData ~* 'ql';

Execute below SELECTs and check Trigram generated result of given string:

SELECT SHOW_TRGM('Anvesh') AS Anvesh;
SELECT SHOW_TRGM('Anvesh Patel') AS AnveshPatel;

Execute below SELECTs and check percentage of similarity:

SELECT SIMILARITY('Anvesh','anv') AS SimilarPercentage;
SELECT SIMILARITY('Anvesh','anve') AS SimilarPercentage;