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 = 266 
WHERE 
  cscart_products_categories.product_id IN (
    387039, 339692, 339737, 339535, 339534, 
    339573, 339496, 339617, 339620, 339621, 
    339742, 339611, 339533, 339710, 81689, 
    339587, 339683, 339444, 170248, 387094, 
    339722, 387042, 339570, 339610, 81674, 
    339584, 339771, 170250, 435270, 339638, 
    159865, 339625, 339706, 339529, 339751, 
    387037, 339736, 173787, 339618, 254333, 
    265275, 387092, 339569, 339581, 339615, 
    159861, 339770, 387040, 339705, 339608, 
    205885, 339619, 254332, 258305, 339783, 
    339693, 391684, 339732, 339779, 339759, 
    339607, 170249, 339572, 159863, 254325, 
    159864, 339775, 170247, 339476, 339589, 
    339652, 339571, 254327, 339778, 391687, 
    387089, 170216, 387056, 173789, 391688, 
    387096, 387093, 170251, 387041, 173763, 
    339738, 339456, 339686, 339550, 391689, 
    387098, 339780, 339739, 387045, 339644, 
    339747, 339586, 339630, 391800, 387044
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00293

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "297.96"
    },
    "grouping_operation": {
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_products_categories",
            "access_type": "range",
            "possible_keys": [
              "PRIMARY",
              "link_type",
              "pt"
            ],
            "key": "pt",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "rows_examined_per_scan": 106,
            "rows_produced_per_join": 106,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "22.36",
              "eval_cost": "21.20",
              "prefix_cost": "43.56",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "link_type"
            ],
            "attached_condition": "(`webmarco`.`cscart_products_categories`.`product_id` in (387039,339692,339737,339535,339534,339573,339496,339617,339620,339621,339742,339611,339533,339710,81689,339587,339683,339444,170248,387094,339722,387042,339570,339610,81674,339584,339771,170250,435270,339638,159865,339625,339706,339529,339751,387037,339736,173787,339618,254333,265275,387092,339569,339581,339615,159861,339770,387040,339705,339608,205885,339619,254332,258305,339783,339693,391684,339732,339779,339759,339607,170249,339572,159863,254325,159864,339775,170247,339476,339589,339652,339571,254327,339778,391687,387089,170216,387056,173789,391688,387096,387093,170251,387041,173763,339738,339456,339686,339550,391689,387098,339780,339739,387045,339644,339747,339586,339630,391800,387044))"
          }
        },
        {
          "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",
              "webmarco.cscart_products_categories.product_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 106,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "106.00",
              "eval_cost": "21.20",
              "prefix_cost": "170.76",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "position"
            ]
          }
        },
        {
          "table": {
            "table_name": "cscart_categories",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "c_status",
              "p_category_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "category_id"
            ],
            "key_length": "3",
            "ref": [
              "webmarco.cscart_products_categories.category_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 5,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "106.00",
              "eval_cost": "1.06",
              "prefix_cost": "297.96",
              "data_read_per_join": "31K"
            },
            "used_columns": [
              "category_id",
              "usergroup_ids",
              "status",
              "storefront_id"
            ],
            "attached_condition": "((`webmarco`.`cscart_categories`.`storefront_id` in (0,1)) and ((`webmarco`.`cscart_categories`.`usergroup_ids` = '') or find_in_set(0,`webmarco`.`cscart_categories`.`usergroup_ids`) or find_in_set(1,`webmarco`.`cscart_categories`.`usergroup_ids`)) and (`webmarco`.`cscart_categories`.`status` in ('A','H')))"
          }
        }
      ]
    }
  }
}