The problem isn't over, yet.

In our development machines (client-servers), where our server has
version 2.0.0.12748 of Firebird, the plan DOES USE the PRIMARY KEY of
table SUBARTIS (when we use the JOINS first and later the OUTER
joins), however, our customer has Firebird version 2.0.1.12855, which
is a newer, BETA version.

The problem I see here is that more of these problems are coming
ahead, since the OLDER VERSION (ours, 2.0.0.12748) IS now using the
correct indexes, but the NEWER version 2.0.1.12855 IS NOT!

I suggested that they should download the older version and now they
can't find it at the download site.

Does anybody have contact with the developers to report such a
nuisance? The main thing is the BUG coming in the future version. To
me, NOT using the correct index if the JOIN is AFTER outer joins, is
NOT proper functioning. BUT not using the correct index AT ALL (as in
version 2.0.1.12855) IS A BUG!

Thanks again for all the great help.

Franz J Fortuny