Hierarchic Self-Referential Data (fixed)

example_postgres.sql

create extension intarray ;

CREATE TABLE messages (
                          created_at  timestamp DEFAULT now(),
                          reply_to    int       REFERENCES messages,
                          id          int       PRIMARY KEY
                              GENERATED BY DEFAULT AS IDENTITY,
                          content     text,
                          CHECK (reply_to <> id)
);
CREATE INDEX ON messages (reply_to, id);

INSERT INTO messages ( reply_to, id, content )
VALUES
    ( null, 1, 'SYN'),
    ( 1,    2, 'SYN-ACK'),
    ( 2,    3, 'ACK'),
    ( null, 4, 'We should give free advertising to PLAN EXPLORER'),
    ( 4,    5, 'Admins should disclose their affiliations with products'),
    ( 5,    6, 'BANNNN'),
    ( 4,    7, 'Plan Explorer only works on one database'),
    ( 7,    8, 'BANNNN'),
    ( 4,    9, 'Plan Explorer does not support Linux'),
    ( 9,   10, 'BANNNN'),
    ( 4,   11, 'Plan Explorer only does what decent databases already do'),
    ( 11,  12, 'BANNNN');

WITH RECURSIVE t(reply_to, id, content, root, level)
                   AS (
        SELECT reply_to, id, content, ARRAY[id], 0
        FROM messages
        WHERE reply_to IS NULL
        UNION ALL
        SELECT messages.reply_to, messages.id, messages.content, root | ARRAY[messages.id], t.level+1
        FROM t
                 JOIN messages
                      ON (messages.reply_to = t.id)
    )
SELECT * FROM t
ORDER BY root;