Pavel,

I can't address your problem directly, but I had a similar issue that
turned out to be a Linux problem. At the time, I was running FB 1.5.1
on a Fedora Core 2 box, w/all the latest Core patches installed. I
was having query problems with a 17M record, 6G database - sometime
queries would simply not finish, other times I was getting strange
error messages from Database Workbench. Anyway, after I upgraded to
Core 3, the query problems went away. FWIW...