There can be only one

[Jason Dusek <jason.dusek@xxxxxxxxx>]

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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