In FB 2.1.2, with some queries I got much slower execution than with 2.1.1. After some investigation, I found that optimizer choose different plan, that is up to 4x slower. I made test database, and here is simple test statement. Maybe something with CORE-2078?

select d.uid
from promene_doc p
  inner join dokumenti d on (p.prom_uid = d.uid)
  inner join tipovi_vrste v on (v.tipvrsta=d.tipvrsta)
where (d.op=2) and (d.stornodok=0)

-- fast 2.1.1 --PLAN JOIN (P NATURAL, D INDEX (DOKUMENTI_PK), V INDEX (TIPOVI_VRSTE_PK))
-- slow 2.1.2 --PLAN JOIN (V NATURAL, D INDEX (FK_DOK_TIPVRSTA), P INDEX (PK_PROMENE_DOC))