SELECT 
  `B`.`brand_id`, 
  `B`.`brand`, 
  `BR`.`current_position`, 
  `PC`.`category_id` 
FROM 
  `cscart_brands` AS `B` 
  LEFT JOIN `brands_rating` AS `BR` ON `BR`.`brand_id` = `B`.`brand_id` 
  AND `BR`.`category_id` = 921 
  AND `BR`.`current_position` > 0 
  LEFT JOIN `cscart_products` AS `P` ON `B`.`brand_id` = `P`.`brand_id` 
  LEFT JOIN `cscart_products_categories` AS `PC` ON `PC`.`product_id` = `P`.`product_id` 
  AND `PC`.`link_type` = 'M' 
WHERE 
  1 
  AND `PC`.`category_id` IN (921) 
  AND `P`.`status` = 'A' 
GROUP BY 
  `B`.`brand_id`, 
  `B`.`brand` 
ORDER BY 
  BR.current_position DESC, 
  `B`.`brand` ASC

Query time 0.00078

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.75"
    },
    "ordering_operation": {
      "using_filesort": true,
      "grouping_operation": {
        "using_temporary_table": true,
        "using_filesort": false,
        "nested_loop": [
          {
            "table": {
              "table_name": "PC",
              "access_type": "ref",
              "possible_keys": [
                "PRIMARY",
                "link_type",
                "pt",
                "idx_pc_product_id_type"
              ],
              "key": "link_type",
              "used_key_parts": [
                "link_type",
                "category_id"
              ],
              "key_length": "6",
              "ref": [
                "const",
                "const"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 1,
              "filtered": "100.00",
              "using_index": true,
              "cost_info": {
                "read_cost": "0.25",
                "eval_cost": "0.10",
                "prefix_cost": "0.35",
                "data_read_per_join": "16"
              },
              "used_columns": [
                "product_id",
                "category_id",
                "link_type"
              ]
            }
          },
          {
            "table": {
              "table_name": "P",
              "access_type": "eq_ref",
              "possible_keys": [
                "PRIMARY",
                "status",
                "cscart_products_brand_id_index",
                "cscart_products_product_id_index",
                "idx_status"
              ],
              "key": "PRIMARY",
              "used_key_parts": [
                "product_id"
              ],
              "key_length": "3",
              "ref": [
                "dev2.PC.product_id"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 0,
              "filtered": "72.23",
              "cost_info": {
                "read_cost": "0.25",
                "eval_cost": "0.07",
                "prefix_cost": "0.70",
                "data_read_per_join": "5K"
              },
              "used_columns": [
                "product_id",
                "status",
                "brand_id"
              ],
              "attached_condition": "((`dev2`.`P`.`status` = 'A') and (`dev2`.`P`.`brand_id` is not null))"
            }
          },
          {
            "table": {
              "table_name": "B",
              "access_type": "eq_ref",
              "possible_keys": [
                "PRIMARY",
                "cscart_brands_pk2"
              ],
              "key": "PRIMARY",
              "used_key_parts": [
                "brand_id"
              ],
              "key_length": "4",
              "ref": [
                "dev2.P.brand_id"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 0,
              "filtered": "100.00",
              "cost_info": {
                "read_cost": "0.18",
                "eval_cost": "0.07",
                "prefix_cost": "0.95",
                "data_read_per_join": "2K"
              },
              "used_columns": [
                "brand_id",
                "brand"
              ]
            }
          },
          {
            "table": {
              "table_name": "BR",
              "access_type": "eq_ref",
              "possible_keys": [
                "unique_brand_category",
                "idx_br_brand_category"
              ],
              "key": "unique_brand_category",
              "used_key_parts": [
                "brand_id",
                "category_id"
              ],
              "key_length": "8",
              "ref": [
                "dev2.P.brand_id",
                "const"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 0,
              "filtered": "100.00",
              "cost_info": {
                "read_cost": "0.72",
                "eval_cost": "0.07",
                "prefix_cost": "1.75",
                "data_read_per_join": "23"
              },
              "used_columns": [
                "id",
                "brand_id",
                "category_id",
                "current_position"
              ],
              "attached_condition": "<if>(is_not_null_compl(BR), (`dev2`.`BR`.`current_position` > 0), true)"
            }
          }
        ]
      }
    }
  }
}