I found that in one table I have dublicated indexes. There is indexes
created by foreign key and another indexes that I created on the same
foreign key fields. I removed these unuseful indexes.
After some days I found out that some of my query execution never
ends. I allmost killed my self trying to find what is wrong.
Finally I added these dublicated indexes back and querys started to
execute in several seconds.

Can anyone explain why this happend? Why firebird is not using foreign
key indexes?