Steps to reproduce:

1. Import template https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/templates/db/postgresql_agent2
2. Create user in PostgreSQL, check the connection
3. Create host in Zabbix - link with Template DB PostgreSQL Agent 2

Result:
All non-dependent items are unsupported

Error in zabbix_agent2.log - see the screenshot

Zabbix_get shows the same error for any item

zabbix_get -s 127.0.0.1 -p 10056 -k pgsql.bgwriter[tcp://127.0.0.1:5432,zbx_monitor,zabbix123]
 ZBX_NOTSUPPORTED: Cannoe Etabilish Connection To Postgres Server: Failed To Connect To `Host=127.0.0.1 User=Zbx_monitor Database=`: Server Error (Fatal: Database  "Zbx_monitor" is not exists (SQLSTATE 3D000)).

Expected:
Correct data

Step for fix

Define database name in item key as the last parameter

Quiestion is - why this is not defined by default?

Defining Plugins.Postgres.Database=<dbname>

in zabbix_agent2.conf is not helping.

Alternative

Set parameters for session in zabbix_agent2.conf

Plugins.Postgres.Sessions.Postgres1.Host=<host>
Plugins.Postgres.Sessions.Postgres1.Port=<port>
Plugins.Postgres.Sessions.Postgres1.User=<UsernameForPostgres1>
Plugins.Postgres.Sessions.Postgres1.Password=<PasswordForPostgres1>
Plugins.Postgres.Sessions.Postgres1.Database=<DatabaseForPostgres1>

Next

Problem with item values in LLD

LLD pgsql.db.discovery["\\\\\\\\{$PG.URI}"]

Item prototype (by plugin description in https://git.zabbix.com/projects/ZBX/repos/zabbix/browse/src/go/plugins/postgres)

pgsql.db.age[uri,username,password,dbName] — age of the oldest xid for each database. Used in databases discovery. Params: dbName — Database name. Mandatory.

Used SQL query

SELECT age(datfrozenxid)
FROM pg_catalog.pg_database
WHERE datistemplate = false
AND datname = <dbName>

 All values for each database return the same value, but databasese were created in the different time

Why this item called as "Database age"?

Item prototype

 pgsql.db.size[uri,username,password,dbName] — database size in bytes. Used in databases discovery. Params: dbName — Database name. Mandatory.

when plugin Session parameters are defined in zabbix_agent2.conf

return the same values for each database

But databases have a different size

Next

Mistake in SQL query in plugin description:

 pgsql.db.discovery[uri,username,password,dbName] — Databases discovery. Params: dbName — Database name. Optional.

Returns: Result of the

SELECT age(datfrozenxid)
FROM pg_catalog.pg_database
WHERE datistemplate = false
AND datname = <dbName>

 query have selecting result from pg_database table without defining any alias - there is a mistake with selecting column d.datname

Please, fix it