SELECT 
  csscart_products_categories.product_id, 
  GROUP_CONCAT(
    IF(
      csscart_products_categories.link_type = "M", 
      CONCAT(
        csscart_products_categories.category_id, 
        "M"
      ), 
      csscart_products_categories.category_id
    )
  ) AS category_ids, 
  product_position_source.position AS position 
FROM 
  csscart_products_categories 
  INNER JOIN csscart_categories ON csscart_categories.category_id = csscart_products_categories.category_id 
  AND csscart_categories.storefront_id IN (0, 1) 
  AND (
    csscart_categories.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, csscart_categories.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, csscart_categories.usergroup_ids
    )
  ) 
  AND csscart_categories.status IN ('A', 'H') 
  LEFT JOIN csscart_products_categories AS product_position_source ON csscart_products_categories.product_id = product_position_source.product_id 
  AND product_position_source.category_id = 171 
WHERE 
  csscart_products_categories.product_id IN (205, 208, 206) 
GROUP BY 
  csscart_products_categories.product_id

Query time 0.00051

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "14.41"
    },
    "grouping_operation": {
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "csscart_products_categories",
            "access_type": "range",
            "possible_keys": [
              "PRIMARY",
              "pt"
            ],
            "key": "pt",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "rows_examined_per_scan": 3,
            "rows_produced_per_join": 3,
            "filtered": "100.00",
            "index_condition": "(`demoecarter_pos`.`csscart_products_categories`.`product_id` in (205,208,206))",
            "cost_info": {
              "read_cost": "6.61",
              "eval_cost": "0.60",
              "prefix_cost": "7.21",
              "data_read_per_join": "48"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "link_type"
            ]
          }
        },
        {
          "table": {
            "table_name": "product_position_source",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "pt"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "category_id",
              "product_id"
            ],
            "key_length": "6",
            "ref": [
              "const",
              "demoecarter_pos.csscart_products_categories.product_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 3,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "3.00",
              "eval_cost": "0.60",
              "prefix_cost": "10.81",
              "data_read_per_join": "48"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "position"
            ]
          }
        },
        {
          "table": {
            "table_name": "csscart_categories",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "c_status",
              "p_category_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "category_id"
            ],
            "key_length": "3",
            "ref": [
              "demoecarter_pos.csscart_products_categories.category_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "3.00",
              "eval_cost": "0.03",
              "prefix_cost": "14.41",
              "data_read_per_join": "400"
            },
            "used_columns": [
              "category_id",
              "storefront_id",
              "usergroup_ids",
              "status"
            ],
            "attached_condition": "((`demoecarter_pos`.`csscart_categories`.`storefront_id` in (0,1)) and ((`demoecarter_pos`.`csscart_categories`.`usergroup_ids` = '') or find_in_set(0,`demoecarter_pos`.`csscart_categories`.`usergroup_ids`) or find_in_set(1,`demoecarter_pos`.`csscart_categories`.`usergroup_ids`)) and (`demoecarter_pos`.`csscart_categories`.`status` in ('A','H')))"
          }
        }
      ]
    }
  }
}