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 = 303 
WHERE 
  cscart_products_categories.product_id IN (
    321, 320, 319, 318, 317, 316, 315, 314, 
    313, 312, 311, 310, 309, 308, 307, 306
  ) 
GROUP BY 
  cscart_products_categories.product_id

Query time 0.00068

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "cscart_products_categories.product_id",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "cscart_categories",
              "access_type": "ALL",
              "possible_keys": ["PRIMARY", "c_status", "p_category_id"],
              "rows": 11,
              "filtered": 100,
              "attached_condition": "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')"
            }
          },
          {
            "table": {
              "table_name": "cscart_products_categories",
              "access_type": "ref",
              "possible_keys": ["PRIMARY", "pt"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["category_id"],
              "ref": ["demov2026.cscart_categories.category_id"],
              "rows": 1,
              "filtered": 30.18867874,
              "index_condition": "cscart_products_categories.product_id in (321,320,319,318,317,316,315,314,313,312,311,310,309,308,307,306)"
            }
          },
          {
            "table": {
              "table_name": "product_position_source",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY", "pt"],
              "key": "PRIMARY",
              "key_length": "6",
              "used_key_parts": ["category_id", "product_id"],
              "ref": ["const", "demov2026.cscart_products_categories.product_id"],
              "rows": 1,
              "filtered": 100
            }
          }
        ]
      }
    }
  }
}