Firebird 2.1 Language Reference Update

DDL statements
Description: From Firebird 2.0 onward view definitions are considered full-fledged SELECT statements. Consequently,
the following elements are (re)allowed in view definitions: FIRST, SKIP, ROWS, ORDER BY, PLAN
and UNION.
Note
The use of a UNION within a view is currently only supported if you supply a column list for the view (this
list is normally optional):
create view vplanes (make, model) as
select make, model from jets
union
select make, model from props
union
select make, model from gliders
In Firebird 2.5, the column list will become optional also for views with UNIONs.
PLAN subclause disallowed in 1.5, reallowed in 2.0
Changed in: 1.5, 2.0
Description: Firebird versions 1.5.x forbid the use of a PLAN subclause in a view definition. From 2.0 onward
a PLAN is allowed again.
Triggers on updatable views block auto-writethrough
Changed in: 2.0
Description: In versions prior to 2.0, Firebird often did not block the automatic writethrough to the underlying
table if one or more triggers were defined on a naturally updatable view. This could cause mutations to be
performed twice unintentionally, sometimes leading to data corruption and other mishaps. Starting at Firebird
2.0, this misbehaviour has been corrected: now if you define a trigger on a naturally updatable view, no mutations
to the view will be automatically passed on to the table; either your trigger takes care of that, or nothing will.
This is in accordance with the description in the InterBase 6 Data Definition Guide under Updating views with
triggers.
Warning
Some people have developed code that counts on or takes advantage of the prior behaviour. Such code should
be corrected for Firebird 2.0 and higher, or mutations may not reach the table at all.
View with non-participating NOT NULL columns in base table can be made
insertable
Changed in: 2.0
Description: Any view whose base table contains one or more non-participating NOT NULL columns is readonly
by nature. It can be made updatable by the use of triggers, but even with those, all INSERT attempts into
such views used to fail because the NOT NULL constraint on the base table was checked before the view trigger
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