Submitted by: @dmitry-starodubov

Is related to QA433

create database 'test.fdb';

set plan on;

CREATE TABLE R$TMP (
POSTING_ID INTEGER
);

CREATE TABLE TMP (
POSTING_ID INTEGER NOT NULL
);

ALTER TABLE TMP ADD CONSTRAINT PK_TMP PRIMARY KEY (POSTING_ID);

select r.POSTING_ID as r$POSTING_ID, t.POSTING_ID from (
SELECT POSTING_ID
FROM r$tmp
) r left join (
select POSTING_ID from
(select
posting_id
from tmp)
) t on r.POSTING_ID = t.POSTING_ID;

drop database;

This SELECT in FB 2.1 and FB-HEAD uses such plan:
PLAN JOIN (R R$TMP NATURAL, T TMP INDEX (PK_TMP))

And FB 2.5 doesn't use primary key index :
PLAN JOIN (R R$TMP NATURAL, T TMP NATURAL)

Commits: 05b983b