How to use full text search in Persian using Postgresql?
use this.

select post_id 
from comments 
where to_tsvector('simple', comments.body) @@ to_tsquery('simple', 'کتاب') ;


OR

select post_id 
from comments 
where to_tsvector('simple', comments.body) @@ to_tsquery('کتاب') ;


Or use this extension:

https://pgroonga.github.io/tutorial/