SELECT 
  SQL_CALC_FOUND_ROWS products.product_id, 
  products.amount, 
  IF(
    shared_descr.product_id IS NOT NULL, 
    shared_descr.product, descr1.product
  ) as product, 
  companies.company as company_name, 
  variation_group_products.group_id AS variation_group_id, 
  products.product_type, 
  products.parent_product_id, 
  1 as instock_marker, 
  IF(
    products.product_type = 'C', 
    (
      SELECT 
        MAX(amount) 
      FROM 
        cscart_products 
      WHERE 
        parent_product_id = products.product_id
    ) > 0, 
    IF(
      products.amount > 0, 
      1, 
      IF(products.tracking = 'O', 1, 0)
    )
  ) as instock_sorting 
FROM 
  cscart_products as products 
  LEFT JOIN cscart_product_descriptions as descr1 ON descr1.product_id = products.product_id 
  AND descr1.lang_code = 'ru' 
  LEFT JOIN cscart_companies AS companies ON companies.company_id = products.company_id 
  INNER JOIN cscart_products_categories as products_categories ON products_categories.product_id = products.product_id 
  INNER JOIN cscart_categories ON cscart_categories.category_id = products_categories.category_id 
  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') 
  AND cscart_categories.storefront_id IN (0, 1) 
  LEFT JOIN cscart_ult_product_descriptions shared_descr ON shared_descr.product_id = products.product_id 
  AND shared_descr.company_id = 1 
  AND shared_descr.lang_code = 'ru' 
  LEFT JOIN cscart_images_links as img_links ON img_links.object_id = products.product_id 
  AND img_links.object_type = 'product' 
  AND img_links.type = 'M' 
  INNER JOIN cscart_product_variation_group_products AS variation_group_products ON variation_group_products.product_id = products.product_id 
WHERE 
  1 
  AND companies.status IN ('A') 
  AND (
    products.usergroup_ids = '' 
    OR FIND_IN_SET(0, products.usergroup_ids) 
    OR FIND_IN_SET(1, products.usergroup_ids)
  ) 
  AND products.status IN ('A') 
  AND img_links.pair_id > 0 
  AND variation_group_products.group_id IN (375) 
GROUP BY 
  products.product_id 
ORDER BY 
  instock_sorting DESC, 
  NULL asc

Query time 0.02435

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "if(products.product_type = 'C',(subquery#2) > 0,if(products.amount > 0,1,if(products.tracking = 'O',1,0))) desc",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "companies",
              "access_type": "system",
              "possible_keys": ["PRIMARY"],
              "rows": 1,
              "filtered": 100
            }
          },
          {
            "table": {
              "table_name": "variation_group_products",
              "access_type": "ref",
              "possible_keys": ["PRIMARY", "idx_group_id"],
              "key": "idx_group_id",
              "key_length": "3",
              "used_key_parts": ["group_id"],
              "ref": ["const"],
              "rows": 450,
              "filtered": 100,
              "attached_condition": "variation_group_products.group_id <=> 375"
            }
          },
          {
            "table": {
              "table_name": "products",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY", "status"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["product_id"],
              "ref": ["lillyshop_lillyshop.variation_group_products.product_id"],
              "rows": 1,
              "filtered": 90.50141144,
              "attached_condition": "products.company_id = 1 and (products.usergroup_ids = '' or find_in_set(0,products.usergroup_ids) or find_in_set(1,products.usergroup_ids)) and products.`status` = 'A'"
            }
          },
          {
            "table": {
              "table_name": "shared_descr",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY", "product_id", "company_id"],
              "key": "PRIMARY",
              "key_length": "13",
              "used_key_parts": ["product_id", "lang_code", "company_id"],
              "ref": [
                "lillyshop_lillyshop.variation_group_products.product_id",
                "const",
                "const"
              ],
              "rows": 1,
              "filtered": 100,
              "attached_condition": "trigcond(shared_descr.lang_code = 'ru')"
            }
          },
          {
            "table": {
              "table_name": "descr1",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY", "product_id"],
              "key": "PRIMARY",
              "key_length": "9",
              "used_key_parts": ["product_id", "lang_code"],
              "ref": [
                "lillyshop_lillyshop.variation_group_products.product_id",
                "const"
              ],
              "rows": 1,
              "filtered": 100,
              "attached_condition": "trigcond(descr1.lang_code = 'ru')"
            }
          },
          {
            "table": {
              "table_name": "products_categories",
              "access_type": "ref",
              "possible_keys": ["PRIMARY", "pt"],
              "key": "pt",
              "key_length": "3",
              "used_key_parts": ["product_id"],
              "ref": ["lillyshop_lillyshop.variation_group_products.product_id"],
              "rows": 1,
              "filtered": 100
            }
          },
          {
            "table": {
              "table_name": "cscart_categories",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY", "c_status", "p_category_id"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["category_id"],
              "ref": ["lillyshop_lillyshop.products_categories.category_id"],
              "rows": 1,
              "filtered": 100,
              "attached_condition": "(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') and cscart_categories.storefront_id in (0,1)"
            }
          },
          {
            "table": {
              "table_name": "img_links",
              "access_type": "ref",
              "possible_keys": ["PRIMARY", "object_id"],
              "key": "object_id",
              "key_length": "81",
              "used_key_parts": ["object_id", "object_type", "type"],
              "ref": [
                "lillyshop_lillyshop.variation_group_products.product_id",
                "const",
                "const"
              ],
              "rows": 1,
              "filtered": 100,
              "index_condition": "img_links.object_id = variation_group_products.product_id and img_links.object_type = 'product' and img_links.`type` = 'M'",
              "attached_condition": "img_links.pair_id > 0"
            }
          }
        ],
        "subqueries": [
          {
            "expression_cache": {
              "state": "uninitialized",
              "query_block": {
                "select_id": 2,
                "nested_loop": [
                  {
                    "table": {
                      "table_name": "cscart_products",
                      "access_type": "ALL",
                      "possible_keys": ["idx_parent_product_id"],
                      "rows": 11707,
                      "filtered": 100,
                      "attached_condition": "cscart_products.parent_product_id = products.product_id"
                    }
                  }
                ]
              }
            }
          }
        ]
      }
    }
  }
}