Steps to reproduce:

1. Install postgresql 12
2. Add official postgresql monitoring https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/templates/db/postgresql

Result:

In postgresql log we see constantly

2020-06-24 17:21:22 EEST ERROR: field name must not be null
2020-06-24 17:21:22 EEST STATEMENT: SELECT json_object_agg(datname, row_to_json(T)) FROM (
 SELECT datname,
 numbackends,
 xact_commit,
 xact_rollback,
 blks_read,
 blks_hit,
 tup_returned,
 tup_fetched,
 tup_inserted,
 tup_updated,
 tup_deleted,
 conflicts,
 temp_files,
 temp_bytes,
 deadlocks
 FROM pg_stat_database) T

Expected:
no entries in the logs

The reason is that in Postgresql 12 the pg_stat_database
has one more line that is agregate for all databases

The pg_stat_database view will contain one row for each database in the cluster, plus one for the shared objects, showing database-wide statistics.

The fix is prety easy ... just add WHERE datname IS NOT NULL in pgsql.dbstat.sql