I'm aware of that and in my case I don't think it will be a problem. It is for a type-ahead search web interface so actually it only requires indexing all possible substrings starting from char 1, ie. p, po, pos, post, postg, postgr, postgre, postgres, postgresq, postgresql.
If you want to provide autocompletion, you could build a list of unique lexemes from inserted posts, and use a btree index and LIKE...
