Excerpts from Darren Duncan's message of mié may 04 15:33:33 -0300 2011:
> I see VARIANT/ANYTYPE as the most general case of supporting union types, which,
> say, could have more specific examples of "allow any number or date here but
> nothing else". If VARIANT is supported, unions in general ought to be also.
Okay, so aside from the performance (storage reduction) gained, there's
this argument for having variant/union types. It seems to me that this
is indeed possible to build. Completely general VARIANT, though, is
rather complex. A declared union, where you specify exactly which types
can be part of the union, can be catalogued, so that the system knows
exactly where to look when a type needs to be modified. A general
VARIANT however looks complex to me to solve.
The problem is this: if an user attempts to drop a type, and this type
is used in a variant somewhere, we would lose the stored data. So the
drop needs to be aborted. Similarly, if we alter a type (easy example:
a composite type) used in a variant, we need to cascade to modify all
rows using that composite.
If the unions that use a certain type are catalogued, we at least know
what tables to scan to cascade.
In a general variant, the system catalogs do not have the information of
what type each variant masquerades as. We would need to examine the
variant's masqueraded types on each insert; if the current type is not
found, add it. This seems a bit expensive.
--
Álvaro Herrera <alvherre@commandprompt.com>
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