Firebird query performance with text search, also some numbers
Nothing suspicious in the plan itself, though I do wonder what your actual query looks like. If KONTRAHENCI.kt_nazwa has an index and there also is an index for the field in OFERTY that links to the primary key field of KONTRAHENCI, then a better plan might be something like:
PLAN JOIN (JOIN (KL INDEX (KT_NAZWA_INDEX), OFERTY INDEX (LINK_FIELD_KONTRAHENCI...

Are you by any chance not using INNER JOINs, but LEFT or RIGHT [OUTER] JOINs? That would prevent Firebird from choosing the optimal index.