Firebird 5.0.1 is not using index when all records are null and left join is converted to inner join

[dsonda]

Given the query below:

SELECT
ITEM_NOTA_COMPRA.INC_ID
FROM ITEM_NOTA_COMPRA
LEFT JOIN NOTA_COMPRA
ON (ITEM_NOTA_COMPRA.NTC_ID = NOTA_COMPRA.NTC_ID)
WHERE (ITEM_NOTA_COMPRA.INC_ID_COMPLEMENTADO = 651321)
AND (NOTA_COMPRA.NTC_SITUACAO = 'Finalizada')

Field INC_ID_COMPLEMENTADO is a foreign key and all record values are null (index stats = 1).

Firebird 2.5.9 uses the FK index from INC_ID_COMPLEMENTADO field:

PLAN JOIN (ITEM_NOTA_COMPRA INDEX (FK_INC_INC_COMPLEMENTADO), NOTA_COMPRA INDEX (PK_NOTA_COMPRA))
Records fetched = 0
Prepare time = 0ms
Execute time = 0ms
Reads from disk to cache = 0
Fetches from cache = 24

Firebird 5.0.1 converts to inner join and do a full scan on right table:

PLAN JOIN (NOTA_COMPRA NATURAL, ITEM_NOTA_COMPRA INDEX (FK_ITEM_NTC_NOTA_COMPRA))
Records fetched = 0
Prepare time = 0ms
Execute time = 1s 0ms
Reads from disk to cache = 13.592
Fetches from cache = 871.441

With "OuterJoinConversion = false", they use the same plan.

Workaround:
Create an "is not null" partial index on field INC_ID_COMPLEMENTADO (stats = 0).
But, then you have two indexes on same field, because it's a foreign key.