Query stuck in mattermost postgresql communication

Ubuntu 18.04PostgreSQL 10.10Mattermost 5.15

On mattermost startup process stay forever on

{"level":"info","ts":1570609055.2422519,"caller":"sqlstore/supplier.go:223","msg":"Pinging SQL master database"}

Running this query

SELECT pid, age(clock_timestamp(), query_start), usename, query, stateFROM pg_stat_activityWHERE query != '' AND query NOT ILIKE '%pg_stat_activity%'ORDER BY query_start desc;

Gives me

 pid  |      age       | usename |         query         | state------+----------------+---------+------------------------------- 8700 | 00:08:54.84669 | mmuser  | SELECT * FROM Systems | idle

From there, how do I diagnose what makes this query never end ?