CREATE TABLE "log".payments
(
  log_id integer NOT NULL DEFAULT nextval('log.log_id_seq'::regclass),
  "time" timestamp with time zone NOT NULL DEFAULT now(),
  deposit boolean NOT NULL,
  username text NOT NULL,
  currency_id text NOT NULL, -- разные валюты могут быть в платежах
  amount numeric NOT NULL, -- Не включает в себя сборы (fee_amount)
  fee_amount numeric NOT NULL DEFAULT  0 ,
  merchant_fee_amount numeric NOT NULL DEFAULT  0 ,
  merchant text,
  CONSTRAINT deposits_withdraws_pkey PRIMARY KEY (log_id),
  CONSTRAINT deposits_withdraws_username_fkey FOREIGN KEY (username)
      REFERENCES users.users (username) MATCH SIMPLE
      ON UPDATE CASCADE ON DELETE SET NULL
);

CREATE OR REPLACE RULE "50_credit_debit" AS
    ON INSERT TO log.payments DO  UPDATE users.funds f SET amount = f.amount + 
        CASE
            WHEN new.deposit THEN new.amount
            ELSE - (new.amount + new.fee_amount)
        END
  WHERE f.username = new.username AND f.currency_id = new.currency_id;
COMMENT ON RULE "50_credit_debit" ON "log".payments IS 'Начисление/списание средств пользователей';

CREATE TABLE users.funds
(
  fund_id serial NOT NULL,
  username text NOT NULL,
  currency_id text NOT NULL,
  amount numeric NOT NULL DEFAULT  0 ,
  CONSTRAINT funds_pkey PRIMARY KEY (fund_id),
  CONSTRAINT funds_currency_fkey FOREIGN KEY (currency_id)
      REFERENCES rates.currencies (currency_id) MATCH SIMPLE
      ON UPDATE CASCADE ON DELETE CASCADE,
  CONSTRAINT funds_username_fkey FOREIGN KEY (username)
      REFERENCES users.users (username) MATCH SIMPLE
      ON UPDATE CASCADE ON DELETE CASCADE,
  CONSTRAINT funds_username_key UNIQUE (username, currency_id),
  CONSTRAINT "100_amount_not_less_than_0" CHECK (NOT amount <  0 ::numeric)
);

insert into "log".payments
(deposit, username, currency_id, amount)
values
(true, 'Вася', 'MMM',  150 ), -- 'МММ' это название тестовой валюты
(true, 'Вася', 'MMM',  50 )

CREATE OR REPLACE RULE "50_credit_debit" AS
    ON INSERT TO log.payments DO  

UPDATE users.funds f SET amount = f.amount + 
        CASE
            WHEN new.deposit THEN new.amount
            ELSE - (new.amount + new.fee_amount)
        END

FROM payments new -- только новые строки!
WHERE f.username = new.username AND f.currency_id = new.currency_id;