How to create n-gram in postgresql

Question

I want search functionality in my application.

I used trigram for achieving it and it is working fine.

Problem is:

trigram is creating sequence of 3 character group of word. I want more than 3 character in single object. For Example:

select show_trgm('abcpqrs');

This will return: {" a"," ab","abc","bcp","cpq","pqr","qrs","rs "}

I want {" a"," ab","abc","bcp","cpq","pqr","qrs","rs ","abcd","bcpq","cpqr"...}

How can I achieve this?

Answer 1

Try this function.

CREATE OR REPLACE FUNCTION myngram(mystr TEXT, n INT) RETURNS TEXT[]
AS $$
DECLARE
str VARCHAR;
arr TEXT[];
BEGIN

  str := lpad(mystr, n - 1 + char_length(mystr), ' ');
  str := rpad(str, n - 1 + char_length(str), ' ');

  arr := array[]::TEXT[];
  FOR i IN 1 .. char_length(str) - n + 1 LOOP
    arr := arr || substring(str from i for n);
  END LOOP;
  RETURN arr;
END
$$
LANGUAGE plpgsql;

Results from my testing.

testdb=# SELECT myngram('abcpqrs', 4);
                             myngram                             
-----------------------------------------------------------------
 {"   a","  ab"," abc",abcp,bcpq,cpqr,pqrs,"qrs ","rs  ","s   "}
(1 row)

testdb=# SELECT myngram('abcpqrs', 5);
                                       myngram                                       
-------------------------------------------------------------------------------------
 {"    a","   ab","  abc"," abcp",abcpq,bcpqr,cpqrs,"pqrs ","qrs  ","rs   ","s    "}
(1 row)