How to store site wide settings in a database?

Question

I\'m debating three different approaches to to storing sitewide settings for a web application.

A key/value pair lookup table, each key represents a setting.

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Answer 1

Following Mike's idea, here is a script to create a table to save pairs of key/value. This integrates a mechanism (constraint) to check that the values is ok with respect to min/max/not null, and it also automatically creates a function fn_setting_XXXX() to quickly get the value of the corresponding setting (correctly casted).

CREATE TABLE settings
(
   id serial,
   name varchar(30),
   type regtype,
   value text,
   v_min double precision,
   v_max double precision,
   v_not_null boolean default true,
   description text,
   constraint settings_pkey primary key(id),
   constraint setting_unique unique(name),
   constraint setting_type check (type in ('boolean'::regtype, 'integer'::regtype, 'double precision'::regtype, 'text'::regtype))
);

/* constraint to check value */

ALTER TABLE settings
  ADD CONSTRAINT check_value
  CHECK (
        case when type in ('integer'::regtype,'double precision'::regtype) then
            case when v_max is not null and v_min is not null then
                value::double precision <= v_max and value::double precision >= v_min
            when v_max is not null then
                value::double precision <= v_max 
            when v_min is not null then
                value::double precision >= v_min
            else
                true
            end
        else
            true
        end
    and
        case when v_not_null then
            value is not null
        else
            true
        end
    );

/* trigger to create get function for quick access to the setting */

CREATE OR REPLACE FUNCTION ft_setting_create_fn_get() RETURNS TRIGGER AS
    $BODY$
    BEGIN
        IF TG_OP <> 'INSERT' THEN
            EXECUTE format($$DROP FUNCTION IF EXISTS fn_setting_%1$I();$$, OLD.name);
        END IF;
        IF TG_OP <> 'DELETE' THEN
            EXECUTE format($$
                CREATE FUNCTION fn_setting_%1$I() RETURNS %2$s AS
                    'SELECT value::%2$s from settings where name = ''%1$I''' language sql
            $$, NEW.name, NEW.type::regtype );
        END IF;
        RETURN NEW;
    END;
    $BODY$
    LANGUAGE plpgsql;


CREATE TRIGGER tr_setting_create_fn_get_insert
    BEFORE INSERT OR DELETE ON settings
    FOR EACH ROW
    EXECUTE PROCEDURE ft_setting_create_fn_get();
COMMENT ON TRIGGER tr_setting_create_fn_get_insert ON settings IS 'Trigger: automatically create get function for inserted settings';

CREATE TRIGGER tr_setting_create_fn_get_update
    BEFORE UPDATE OF type, name ON settings
    FOR EACH ROW
    WHEN ( NEW.type <> OLD.type OR  OLD.name <> NEW.name)
    EXECUTE PROCEDURE ft_setting_create_fn_get();
COMMENT ON TRIGGER tr_setting_create_fn_get_update ON settings IS 'Trigger: automatically create get function for inserted settings';