SELECT 
  object_id 
FROM 
  cscart_settings_objects 
  LEFT JOIN cscart_settings_sections ON cscart_settings_objects.section_id = cscart_settings_sections.section_id 
WHERE 
  cscart_settings_objects.name = 'watermark'

Query time 0.00030

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "4.80"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_settings_objects",
          "access_type": "ref",
          "possible_keys": [
            "name"
          ],
          "key": "name",
          "used_key_parts": [
            "name"
          ],
          "key_length": "386",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 2,
          "rows_produced_per_join": 2,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "2.00",
            "eval_cost": "0.40",
            "prefix_cost": "2.40",
            "data_read_per_join": "1K"
          },
          "used_columns": [
            "object_id",
            "name",
            "section_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "cscart_settings_sections",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "section_id"
          ],
          "key_length": "4",
          "ref": [
            "eshopuga_upgr_4122.cscart_settings_objects.section_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 2,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "2.00",
            "eval_cost": "0.40",
            "prefix_cost": "4.80",
            "data_read_per_join": "800"
          },
          "used_columns": [
            "section_id"
          ]
        }
      }
    ]
  }
}