Administrace komentářů

• kedze je pouzity typ DATE a predpokladam, ze v jednom dni moze byt viac zaznamov, tak sa tazko urci, ktore 3 z viacerych s rovnakym datumom su tie posledne
• "ponechat aspon 3" je dost vagne - ale zhodou okolnosti to zjednodusi implementaciu tej query - len jej vysledok nebude deterministicky

CREATE TABLE msg_log
(
   username   VARCHAR(100)
 , msg        TEXT(100)
 , created_at DATE
);
CREATE INDEX idx_msg_log_created_at ON msg_log (username, created_at);
INSERT INTO msg_log VALUES ('y', 'A', CURRENT_DATE - interval 6 month);
INSERT INTO msg_log VALUES ('x', 'B', CURRENT_DATE - interval 5 month);
INSERT INTO msg_log VALUES ('x', 'C', CURRENT_DATE - interval 4 month);
INSERT INTO msg_log VALUES ('x', 'D', CURRENT_DATE - interval 4 month);
INSERT INTO msg_log VALUES ('x', 'E', CURRENT_DATE - interval 4 month);
INSERT INTO msg_log VALUES ('x', 'F', CURRENT_DATE - interval 3 month);
INSERT INTO msg_log VALUES ('x', 'G', CURRENT_DATE - interval 2 month);
INSERT INTO msg_log VALUES ('x', 'H', CURRENT_DATE - interval 1 month);
INSERT INTO msg_log VALUES ('x', 'I', CURRENT_DATE - interval 1 day);
INSERT INTO msg_log VALUES ('y', 'J', CURRENT_DATE);
INSERT INTO msg_log VALUES ('z', 'K', CURRENT_DATE - interval 10 month);
INSERT INTO msg_log VALUES ('z', 'L', CURRENT_DATE - interval 9 month);
INSERT INTO msg_log VALUES ('z', 'M', CURRENT_DATE - interval 8 month);
INSERT INTO msg_log VALUES ('z', 'N', CURRENT_DATE - interval 7 month);
INSERT INTO msg_log VALUES ('z', 'O', CURRENT_DATE - interval 6 month);
INSERT INTO msg_log VALUES ('z', 'P', CURRENT_DATE - interval 3 month);

DELETE d
  FROM msg_log AS d
  WHERE d.username = 'z'
    AND d.created_at < (
   SELECT min(x.created_at) min_ts
     FROM (
      SELECT *
         FROM msg_log l
         WHERE l.created_at < CURRENT_DATE - interval 1 month
           AND l.username = 'z'
         ORDER BY l.created_at DESC
         LIMIT 3
   ) x)
;

CREATE TABLE msg_log
(
   username   VARCHAR
 , msg        TEXT
 , created_at DATE
;
CREATE INDEX idx_msg_log_created_at ON msg_log (username, created_at);

INSERT INTO msg_log VALUES ('y', 'A', current_date - interval '6 month');
INSERT INTO msg_log VALUES ('x', 'B', current_date - interval '5 month');
INSERT INTO msg_log VALUES ('x', 'C', current_date - interval '4 month');
INSERT INTO msg_log VALUES ('x', 'D', current_date - interval '4 month');
INSERT INTO msg_log VALUES ('x', 'E', current_date - interval '4 month');
INSERT INTO msg_log VALUES ('x', 'F', current_date - interval '3 month');
INSERT INTO msg_log VALUES ('x', 'G', current_date - interval '2 month');
INSERT INTO msg_log VALUES ('x', 'H', current_date - interval '1 month');
INSERT INTO msg_log VALUES ('x', 'I', current_date - interval '1 day');
INSERT INTO msg_log VALUES ('y', 'J', current_date);
INSERT INTO msg_log VALUES ('z', 'K', current_date - interval '10 month');
INSERT INTO msg_log VALUES ('z', 'L', current_date - interval '9 month');
INSERT INTO msg_log VALUES ('z', 'M', current_date - interval '8 month');
INSERT INTO msg_log VALUES ('z', 'N', current_date - interval '7 month');
INSERT INTO msg_log VALUES ('z', 'O', current_date - interval '6 month');
INSERT INTO msg_log VALUES ('z', 'P', current_date - interval '3 month');

WITH users AS (
-- idealne mat ulozene v tabulke, na ktoru odkazuje cudzi kluc na msg_log(username)
      SELECT DISTINCT username FROM msg_log
   ), maxDateKeptPerUser AS (
      SELECT u.username, (
         SELECT min(x.created_at)
             FROM (SELECT *
                     FROM msg_log l
                     WHERE l.created_at < CURRENT_DATE - interval '1 month'
                       AND l.username = u.username
                     ORDER BY l.created_at DESC
                     LIMIT 3
                  ) x
         ) AS created_at
         FROM users AS u
   )
DELETE FROM msg_log AS d
   USING maxDateKeptPerUser AS m
   WHERE d.username   = m.username
     AND d.created_at < m.created_at
;