Submitted by: @cincuranet

Directly from my thread "Wrong rdb$field_sub_type for numeric computed column" in firebird-devel:

have a table:
recreate table test (
base numeric(15,4),
test1 computed by (base+base),
test2 computed by (cast(base+base as numeric(15,4))),
foo bigint,
bar numeric(15,4)
);

If I run the query:
select rf.RDB$FIELD_NAME, f.RDB$FIELD_SCALE, f.RDB$FIELD_TYPE,
f.RDB$FIELD_SUB_TYPE
from RDB$RELATION_FIELDS rf inner join RDB$FIELDS f on
(rf.RDB$FIELD_SOURCE = f.RDB$FIELD_NAME)
where rf.RDB$RELATION_NAME = 'TEST'
it reports:
BASE -4 16 1
TEST1 -4 16 0
TEST2 -4 16 1
FOO 0 16 0
BAR -4 16 1

Why is rdb$field_sub_type for test1 column "0"? Then it looks like
bigint. Checking the rdb$field_scale could be a solution, but is it
then numeric or decimal?

I'm happy with conclusion that if field_scale is < 0 and no particular
sub_type, imply i.e. numeric. Probably somebody from core devs should
tell us.

That is what is done in Jaybird right now.

See getDataType(...) and getDataTypeName(...) in
http://firebird.cvs.sourceforge.net/viewvc/firebird/client-java/src/main/org/firebirdsql/jdbc/AbstractDatabaseMetaData.java?view=markup

Commits: 35c3ae1