Experimente esta função.
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;
Resultados dos meus testes.
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)
