SELECT 
  SQL_CALC_FOUND_ROWS products.product_id, 
  IF(
    shared_descr.product_id IS NOT NULL, 
    shared_descr.product, descr1.product
  ) as product, 
  companies.company as company_name, 
  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 = 'uk' 
  LEFT JOIN cscart_product_prices as prices ON prices.product_id = products.product_id 
  AND prices.lower_limit = 1 
  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 = 'uk' 
  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' 
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 prices.usergroup_id IN (0, 0, 1) 
  AND img_links.pair_id > 0 
  AND 1 != 1 
GROUP BY 
  products.product_id 
ORDER BY 
  instock_sorting DESC, 
  product asc, 
  products.product_id ASC 
LIMIT 
  0, 100

Query time 0.00093

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "message": "Impossible WHERE"
    },
    "subqueries": [
      {
        "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"
              }
            }
          ]
        }
      }
    ]
  }
}