How to solve 'tuple concurrently updated' error? The following query is performed concurrently by two threads logged in with two different users:

WITH raw_stat AS (
    SELECT
       host(client_addr) as client_addr,
       pid ,
       usename
    FROM pg_stat_activity
    WHERE usename = current_user
)
INSERT INTO my_stat(id, client_addr, pid, usename)
    SELECT nextval('mystat_sequence'), t.client_addr, t.pid, t.usename
    FROM (
        SELECT client_addr, pid, usename
        FROM raw_stat s
        WHERE
            NOT EXISTS (
               SELECT NULL
               FROM my_stat u
               WHERE current_date = u.creation
                   AND s.pid = u.pid
                   AND s.client_addr = u.client_addr
                   AND s.usename = u.usename
            )
    ) t;


From time to time, I get the following error:

tuple concurrently updated


I can't figure out what throw  this error and why this error is thrown. Can you shed a light ?



Here is the sql definition of the table my_stat.

CREATE TABLE my_stat
(
  id bigint NOT NULL,
  creation date NOT NULL DEFAULT current_date,

  client_addr text NOT NULL,
  pid integer NOT NULL,
  usename name NOT NULL,
  CONSTRAINT mystat_pkey PRIMARY KEY (id)
)
WITH (
  OIDS=FALSE
);
I manage to solve my problem by changing my query to this one:

INSERT INTO my_stat(id, client_addr, pid, usename)
    SELECT nextval('mystat_sequence'), client_addr, pid, usename
    FROM (
        SELECT
           host(client_addr) as client_addr,
           pid ,
           usename
        FROM pg_stat_activity
        WHERE usename = current_user
    ) s
    WHERE
        NOT EXISTS (
           SELECT NULL
           FROM my_stat u
           WHERE current_date = u.creation
               AND s.pid = u.pid
               AND s.client_addr = u.client_addr
               AND s.usename = u.usename
        );


I think something happened under the hood right from the Postgresql internals but I can't figure out what ...