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 = 265 
WHERE 
  cscart_products_categories.product_id IN (
    6963, 6952, 6953, 3484, 3487, 3485, 3486, 
    6969, 3434, 6947, 3424, 3911, 3422, 
    3910, 3421, 3909, 3420, 3423, 3912, 
    3430, 3429, 3437, 3436, 3438, 6959, 
    3425, 6958, 6960, 6968, 6961, 6964, 
    6957, 6956, 6954, 6955, 3500, 3501, 
    3431, 6948, 3499, 3914, 3913, 3455, 
    3466, 3454, 3456, 3449, 3461, 3441, 
    3444, 3471, 3442, 3480, 3448, 3440, 
    3467, 3468, 3435, 3476, 3447, 3439, 
    3465, 3446, 3460, 3443, 3469, 3464, 
    3459, 6950, 3470, 3457, 3462, 3450, 
    3458, 3463, 3451, 3445, 3472, 3481, 
    3482, 3475, 3483, 3474, 3473, 3426, 
    6949, 3427, 3477, 3452, 3478, 3479, 
    3453, 3491, 3488, 3497, 3495
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00273

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "69.79"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "2.59"
      },
      "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": "ref",
            "possible_keys": [
              "PRIMARY",
              "pt"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "category_id"
            ],
            "key_length": "3",
            "ref": [
              "gaseus.cscart_categories.category_id"
            ],
            "rows_examined_per_scan": 110,
            "rows_produced_per_join": 2,
            "filtered": "1.47",
            "index_condition": "(`gaseus`.`cscart_products_categories`.`product_id` in (6963,6952,6953,3484,3487,3485,3486,6969,3434,6947,3424,3911,3422,3910,3421,3909,3420,3423,3912,3430,3429,3437,3436,3438,6959,3425,6958,6960,6968,6961,6964,6957,6956,6954,6955,3500,3501,3431,6948,3499,3914,3913,3455,3466,3454,3456,3449,3461,3441,3444,3471,3442,3480,3448,3440,3467,3468,3435,3476,3447,3439,3465,3446,3460,3443,3469,3464,3459,6950,3470,3457,3462,3450,3458,3463,3451,3445,3472,3481,3482,3475,3483,3474,3473,3426,6949,3427,3477,3452,3478,3479,3453,3491,3488,3497,3495))",
            "cost_info": {
              "read_cost": "44.00",
              "eval_cost": "0.26",
              "prefix_cost": "66.30",
              "data_read_per_join": "41"
            },
            "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",
              "gaseus.cscart_products_categories.product_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 2,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "0.65",
              "eval_cost": "0.26",
              "prefix_cost": "67.21",
              "data_read_per_join": "41"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "position"
            ]
          }
        }
      ]
    }
  }
}