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 (
    1646, 1647, 1648, 1649, 1650, 1651, 1652, 
    5326, 5327, 5328, 5329, 5330, 5331, 
    5332, 5333, 5334, 5335, 5336, 5337, 
    5338, 5339, 5340, 5341, 5342, 5343, 
    5344, 5345, 5346, 5347, 5348, 5349, 
    5350, 5351, 5352, 2832, 2833, 2834, 
    2835, 6417, 6418, 6419, 6420, 6421, 
    2823, 2824, 2825, 2826, 6408, 6409, 
    6410, 6411, 6412, 2827, 2828, 2829, 
    2830, 2831, 6413, 6414, 6415, 6416, 
    2928, 2929, 2930, 2931, 2932, 6513, 
    6514, 6515, 6516, 3032, 6609, 3028, 
    3029, 3030, 3031, 6605, 6606, 6607, 
    6608, 6610, 6611, 6612, 6613, 3024, 
    3025, 3026, 3027, 3301, 6847, 3300, 
    2841, 2842, 2843, 2844, 6426
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00289

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "68.52"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "1.64"
      },
      "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": 1,
            "filtered": "0.93",
            "index_condition": "(`gaseus`.`cscart_products_categories`.`product_id` in (1646,1647,1648,1649,1650,1651,1652,5326,5327,5328,5329,5330,5331,5332,5333,5334,5335,5336,5337,5338,5339,5340,5341,5342,5343,5344,5345,5346,5347,5348,5349,5350,5351,5352,2832,2833,2834,2835,6417,6418,6419,6420,6421,2823,2824,2825,2826,6408,6409,6410,6411,6412,2827,2828,2829,2830,2831,6413,6414,6415,6416,2928,2929,2930,2931,2932,6513,6514,6515,6516,3032,6609,3028,3029,3030,3031,6605,6606,6607,6608,6610,6611,6612,6613,3024,3025,3026,3027,3301,6847,3300,2841,2842,2843,2844,6426))",
            "cost_info": {
              "read_cost": "44.00",
              "eval_cost": "0.16",
              "prefix_cost": "66.30",
              "data_read_per_join": "26"
            },
            "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": 1,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "0.41",
              "eval_cost": "0.16",
              "prefix_cost": "66.88",
              "data_read_per_join": "26"
            },
            "used_columns": [
              "product_id",
              "category_id",
              "position"
            ]
          }
        }
      ]
    }
  }
}