Create a partial unique index on is_default. Am 11. Oktober 2015 09:41:08 MESZ, schrieb Jason Dusek <jason.du...@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. >​
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