SELECT 
  cscart_products_categories.product_id, 
  GROUP_CONCAT(
    IF(
      cscart_products_categories.link_type = "M", 
      CONCAT(
        cscart_products_categories.category_id, 
        "M"
      ), 
      cscart_products_categories.category_id
    )
  ) AS category_ids, 
  product_position_source.position AS position 
FROM 
  cscart_products_categories 
  INNER JOIN cscart_categories ON cscart_categories.category_id = cscart_products_categories.category_id 
  AND cscart_categories.storefront_id IN (0, 1) 
  AND (
    cscart_categories.usergroup_ids = '' 
    OR FIND_IN_SET(
      0, cscart_categories.usergroup_ids
    ) 
    OR FIND_IN_SET(
      1, cscart_categories.usergroup_ids
    )
  ) 
  AND cscart_categories.status IN ('A', 'H') 
  LEFT JOIN cscart_products_categories AS product_position_source ON cscart_products_categories.product_id = product_position_source.product_id 
  AND product_position_source.category_id = 271 
WHERE 
  cscart_products_categories.product_id IN (
    6344, 6345, 6346, 6347, 2818, 2819, 2820, 
    2821, 2822, 6404, 6405, 6406, 6407, 
    2933, 2934, 2935, 2936, 6517, 6518, 
    6519, 6520, 6521, 2510, 2511
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00083

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "37.34"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "4.00"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_categories",
            "access_type": "ALL",
            "possible_keys": [
              "PRIMARY",
              "c_status",
              "p_category_id"
            ],
            "rows_examined_per_scan": 40,
            "rows_produced_per_join": 1,
            "filtered": "4.00",
            "cost_info": {
              "read_cost": "4.54",
              "eval_cost": "0.16",
              "prefix_cost": "4.70",
              "data_read_per_join": "6K"
            },
            "used_columns": [
              "category_id",
              "storefront_id",
              "usergroup_ids",
              "status"
            ],
            "attached_condition": "((`gaseus`.`cscart_categories`.`storefront_id` in (0,1)) and ((`gaseus`.`cscart_categories`.`usergroup_ids` = '') or (0 <> find_in_set(0,`gaseus`.`cscart_categories`.`usergroup_ids`)) or (0 <> find_in_set(1,`gaseus`.`cscart_categories`.`usergroup_ids`))) and (`gaseus`.`cscart_categories`.`status` in ('A','H')))"
          }
        },
        {
          "table": {
            "table_name": "cscart_products_categories",
            "access_type": "range",
            "possible_keys": [
              "PRIMARY",
              "pt"
            ],
            "key": "pt",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "rows_examined_per_scan": 25,
            "rows_produced_per_join": 4,
            "filtered": "10.00",
            "index_condition": "(`gaseus`.`cscart_products_categories`.`product_id` in (6344,6345,6346,6347,2818,2819,2820,2821,2822,6404,6405,6406,6407,2933,2934,2935,2936,6517,6518,6519,6520,6521,2510,2511))",
            "using_join_buffer": "hash join",
            "cost_info": {
              "read_cost": "23.62",
              "eval_cost": "0.40",
              "prefix_cost": "32.32",
              "data_read_per_join": "64"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "link_type"
            ],
            "attached_condition": "(`gaseus`.`cscart_products_categories`.`category_id` = `gaseus`.`cscart_categories`.`category_id`)"
          }
        },
        {
          "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",
              "gaseus.cscart_products_categories.product_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 4,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "0.63",
              "eval_cost": "0.40",
              "prefix_cost": "33.34",
              "data_read_per_join": "64"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "position"
            ]
          }
        }
      ]
    }
  }
}