On 05/06/2011 04:08 PM, Alvaro Herrera wrote:
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.
So how is a declared union going to look and operate? Something like this?
CREATE TYPE foo AS UNION (ival int, tval text, tsval timestamptz):
CREATE TABLE bar (myunion foo);
INSERT INTO bar (myunion) VALUES (ival=>1), (tval=>'some text');
UPDATE bar SET myunion.tsval = CURRENT_TIMESTAMP;
Something like that could actually be quite nice for a number of purposes.
cheers
andrew
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