SELECT 
  s.status 
FROM 
  cscart_statuses AS s 
  JOIN cscart_status_descriptions AS sd ON sd.status_id = s.status_id 
  JOIN cscart_status_data AS sda ON sda.status_id = s.status_id 
  AND sda.param = 'inventory' 
  AND sda.value = 'I' 
WHERE 
  s.type = 'O' 
GROUP BY 
  s.status

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "3.11"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "sda",
            "access_type": "ref",
            "possible_keys": [
              "PRIMARY",
              "inventory"
            ],
            "key": "inventory",
            "used_key_parts": [
              "value"
            ],
            "key_length": "765",
            "ref": [
              "const"
            ],
            "rows_examined_per_scan": 8,
            "rows_produced_per_join": 0,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "2.00",
              "eval_cost": "0.08",
              "prefix_cost": "2.80",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "status_id",
              "param",
              "value"
            ],
            "attached_condition": "(`demouniformecomd_db`.`sda`.`param` = 'inventory')"
          }
        },
        {
          "table": {
            "table_name": "s",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "status"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "status_id"
            ],
            "key_length": "3",
            "ref": [
              "demouniformecomd_db.sda.status_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "0.20",
              "eval_cost": "0.01",
              "prefix_cost": "3.08",
              "data_read_per_join": "1"
            },
            "used_columns": [
              "status_id",
              "status",
              "type"
            ],
            "attached_condition": "(`demouniformecomd_db`.`s`.`type` = 'O')"
          }
        },
        {
          "table": {
            "table_name": "sd",
            "access_type": "ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "status_id"
            ],
            "key_length": "3",
            "ref": [
              "demouniformecomd_db.sda.status_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.02",
              "eval_cost": "0.01",
              "prefix_cost": "3.11",
              "data_read_per_join": "124"
            },
            "used_columns": [
              "status_id"
            ]
          }
        }
      ]
    }
  }
}