As reported on discord by CurrentDesk:

We created a materialized view, a function to update said view and then created triggers to call said function. The materialized view, function and triggers all work successfully by all appearances, but we get this error in the console.

{
  "statement": "\nselect \n  COALESCE(\n    json_agg(\n      row_to_json(info)\n    ), \n    '[]' :: JSON\n  ) AS tables \nFROM \n  (\n    select \n      ist.table_schema, \n      ist.table_name,\n      ist.table_type,\n      obj_description(\n        (\n          quote_ident(ist.table_schema) || '.' || quote_ident(ist.table_name)\n        ):: regclass, \n        'pg_class'\n      ) AS comment, \n      COALESCE(json_agg(\n        DISTINCT row_to_json(is_columns) :: JSONB || jsonb_build_object(\n          'comment',\n          (\n            SELECT \n              pg_catalog.col_description(\n                c.oid, is_columns.ordinal_position :: int\n              ) \n            FROM \n              pg_catalog.pg_class c \n            WHERE \n              c.oid = (\n                SELECT \n                  (\n                    (\n                      quote_ident(ist.table_schema) || '.' || quote_ident(ist.table_name)\n                    ):: text\n                  ):: regclass :: oid\n              ) \n              AND c.relname = is_columns.table_name\n          )\n        )\n      ) FILTER (WHERE is_columns.column_name IS NOT NULL), '[]' :: JSON) AS columns,\n      COALESCE(json_agg(\n        DISTINCT row_to_json(is_triggers) :: JSONB || jsonb_build_object(\n          'comment',\n          (\n            SELECT description FROM pg_description JOIN pg_trigger ON pg_description.objoid = pg_trigger.oid \n            WHERE tgname = is_triggers.trigger_name\n          )\n        )\n      ) FILTER (WHERE is_triggers.trigger_name IS NOT NULL), '[]' :: JSON) AS triggers,\n      row_to_json(is_views) AS view_info\n    FROM \n      information_schema.tables AS ist \n      LEFT OUTER JOIN information_schema.columns AS is_columns ON \n        is_columns.table_schema = ist.table_schema \n        AND is_columns.table_name = ist.table_name \n      LEFT OUTER JOIN information_schema.views AS is_views ON is_views.table_schema = ist.table_schema\n        AND is_views.table_name = ist.table_name\n      LEFT OUTER JOIN information_schema.triggers AS is_triggers ON \n        is_triggers.event_object_schema = ist.table_schema AND \n        is_triggers.event_object_table = ist.table_name\n    where (ist.table_schema='public') \n    GROUP BY \n      ist.table_schema, \n      ist.table_name,\n      ist.table_type,\n      is_views.*\n  ) AS info\n",
  "prepared": false,
  "error": {
    "exec_status": "FatalError",
    "hint": null,
    "message": "more than one row returned by a subquery used as an expression",
    "status_code": "21000",
    "description": null
  },
  "arguments": []
}

We experienced this issue on beta.6 and have upgraded to beta.9 and the issue hasn't resolved itself.

According to @rakeshkky, the query is generated by the console; he can probably provide more details than I can.