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 (
    391789, 265231, 81619, 285940, 81632, 
    271489, 81662, 339530, 265039, 265034, 
    285937, 271497, 387095, 81615, 392212, 
    285942, 339446, 271493, 81639, 271504, 
    265258, 81706, 265031, 339514, 264987, 
    265041, 265261, 265263, 339551, 271500, 
    339521, 286476, 81649, 264988, 285945, 
    271506, 265244, 265256, 271496, 271492, 
    339501, 151897, 339499, 285939, 339495, 
    339721, 366887, 392150, 81622, 392214, 
    392215, 265038, 81652, 271498, 81697, 
    339494, 265242, 339503, 339531, 224456, 
    81693, 339516, 285938, 81660, 271510, 
    359818, 285943, 271503, 339539, 285941, 
    339447, 339509, 81669, 271494, 264989, 
    285946, 264990, 271502, 265232, 265254, 
    339636, 285966, 285970, 114755, 339637, 
    339492, 285968, 339538, 369244, 271490, 
    339506, 265250, 265001, 265237, 271505, 
    339724, 392179, 339635, 81628, 339502
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00265

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "317.57"
    },
    "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": 113,
            "rows_produced_per_join": 113,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "23.77",
              "eval_cost": "22.60",
              "prefix_cost": "46.37",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "link_type"
            ],
            "attached_condition": "(`webmarco`.`cscart_products_categories`.`product_id` in (391789,265231,81619,285940,81632,271489,81662,339530,265039,265034,285937,271497,387095,81615,392212,285942,339446,271493,81639,271504,265258,81706,265031,339514,264987,265041,265261,265263,339551,271500,339521,286476,81649,264988,285945,271506,265244,265256,271496,271492,339501,151897,339499,285939,339495,339721,366887,392150,81622,392214,392215,265038,81652,271498,81697,339494,265242,339503,339531,224456,81693,339516,285938,81660,271510,359818,285943,271503,339539,285941,339447,339509,81669,271494,264989,285946,264990,271502,265232,265254,339636,285966,285970,114755,339637,339492,285968,339538,369244,271490,339506,265250,265001,265237,271505,339724,392179,339635,81628,339502))"
          }
        },
        {
          "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": 113,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "113.00",
              "eval_cost": "22.60",
              "prefix_cost": "181.97",
              "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": "113.00",
              "eval_cost": "1.13",
              "prefix_cost": "317.57",
              "data_read_per_join": "33K"
            },
            "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')))"
          }
        }
      ]
    }
  }
}