There can be only one

Create a partial unique index on is_default.

Am 11. Oktober 2015 09:41:08 MESZ, schrieb Jason Dusek <jason.dusek@gmail.com>:

Consider a table of providers, for which one is the default. For
example,
payment providers:

CREATE TABLE payment_via (
id uuid PRIMARY KEY,
provider text NOT NULL,
keys hstore NOT NULL DEFAULT ''
);

Here we store together the name of the provider — medici, paypal — with
access tokens needed to use a certain payment account. How shall we
store
which one is the default? Ideally, we’d be able to ensure there is *but
one*
default.

CREATE TABLE payment_via (
id uuid PRIMARY KEY,
provider text NOT NULL,
keys hstore NOT NULL DEFAULT '',
is_default boolean NOT NULL DEFAULT FALSE
);

How shall we state the constraint? The obvious thing would seem to be:

CREATE TABLE payment_via (
id uuid PRIMARY KEY,
provider text NOT NULL,
keys hstore NOT NULL DEFAULT '',
is_default boolean NOT NULL DEFAULT FALSE,
EXCLUDE (is_default USING AND)
);

However, this is a syntax error. There is always:

CREATE TABLE payment_via (
id uuid PRIMARY KEY,
provider text NOT NULL,
keys hstore NOT NULL DEFAULT '',
is_default boolean NOT NULL DEFAULT FALSE,
EXCLUDE (is_default USING =) WHERE (is_default)
);

but this seems awkward and I was hoping there was some way to use AND
as an
operator.
​

--
Diese Nachricht wurde von meinem Android-Mobiltelefon mit K-9 Mail gesendet.

Andreas Kretschmer <andreas@a-kretschmer.de> wrote:

Create a partial unique index on is_default.

as an example:

test=# CREATE TABLE payment_via (
id int PRIMARY KEY,
provider text NOT NULL,
keys hstore NOT NULL DEFAULT '',
is_default boolean NOT NULL DEFAULT FALSE
);
CREATE TABLE
test=*# create unique index idx_default on payment_via(is_default) where
is_default;
CREATE INDEX
test=*# insert into payment_via values (1, 'foo','', true);
INSERT 0 1
test=*# insert into payment_via values (2, 'bla','', false);
INSERT 0 1
test=*# insert into payment_via values (3, 'blubb','', true);
ERROR: duplicate key value violates unique constraint "idx_default"
DETAIL: Key (is_default)=(t) already exists.
test=*#

Andreas
--
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect. (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly." (unknown)
Kaufbach, Saxony, Germany, Europe. N 51.05082�, E 13.56889�

--
Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general