I don't have InterBase 6, but I tried on Firebird 1.5, and I have to use
`RDB$SYSTEM_FLAG IS NULL` for your query to return non-system indexes,
and that is because the flag is 1 for system, and null for non-system on
that version, and as far as I know that was inherited from InterBase 6.

Hi:

  That worked. I do not understand since the flag is 0 and not null.

This does not set well with me. Raises questions.

Best regards. You say this was "fixed".

So in later version 0 works but does null also work?

Thanks