Postgres 12

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table t (
     login text,
     smth text,
     ts timestamp,
     ts_date date generated always as (ts::date) stored
);

create unique index unq_t_login_timestamp on t(login, ts_date)

CREATE TABLE

CREATE INDEX

INSERT INTO t (login, smth, ts)
    VALUES ('username', 'smth', CURRENT_TIMESTAMP)
    ON CONFLICT (login, ts_date) DO UPDATE
          SET smth = 'smth',
              ts = CURRENT_TIMESTAMP

INSERT 0 1

select * from t

login
smth
ts
ts_date

username
smth
2023-11-20 04:12:35.640667
2023-11-20

SELECT 1

INSERT INTO t (login, smth, ts)
    VALUES ('username', 'smth', CURRENT_TIMESTAMP)
    ON CONFLICT (login, ts_date) DO UPDATE
          SET smth = 'smth',
              ts = CURRENT_TIMESTAMP

INSERT 0 1

select * from t

login
smth
ts
ts_date

username
smth
2023-11-20 04:12:35.642957
2023-11-20

SELECT 1