SELECT 
  SQL_CALC_FOUND_ROWS products.product_id, 
  IF(
    shared_descr.product_id IS NOT NULL, 
    shared_descr.product, descr1.product
  ) as product, 
  SUM(cscart_product_sales.amount) as sales_amount, 
  stoc_intern, 
  pret_achizitie, 
  pret_revanzator, 
  gestiune 
FROM 
  cscart_products as products 
  LEFT JOIN cscart_product_options_inventory as inventory ON inventory.product_id = products.product_id 
  LEFT JOIN cscart_product_descriptions as descr1 ON descr1.product_id = products.product_id 
  AND descr1.lang_code = 'ro' 
  LEFT JOIN cscart_product_prices as prices ON prices.product_id = products.product_id 
  AND prices.lower_limit = 1 
  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') 
  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 = 'ro' 
  INNER JOIN cscart_product_sales ON cscart_product_sales.product_id = products.product_id 
  AND cscart_product_sales.category_id = products_categories.category_id 
WHERE 
  1 
  AND cscart_categories.company_id = 1 
  AND (
    CASE products.tracking WHEN 'O' THEN inventory.amount > 0 WHEN 'B' THEN products.amount > 0 ELSE 1 END
  ) 
  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 cscart_product_sales.amount >= 1 
GROUP BY 
  cscart_product_sales.product_id 
ORDER BY 
  sales_amount desc, 
  product_id ASC 
LIMIT 
  0, 5

Query time 0.46583

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1367.97"
    },
    "ordering_operation": {
      "using_filesort": true,
      "grouping_operation": {
        "using_temporary_table": true,
        "using_filesort": false,
        "nested_loop": [
          {
            "table": {
              "table_name": "inventory",
              "access_type": "system",
              "possible_keys": [
                "pc"
              ],
              "rows_examined_per_scan": 0,
              "rows_produced_per_join": 1,
              "filtered": "0.00",
              "const_row_not_found": true,
              "cost_info": {
                "read_cost": "0.00",
                "eval_cost": "0.10",
                "prefix_cost": "0.00",
                "data_read_per_join": "880"
              },
              "used_columns": [
                "product_id",
                "amount"
              ]
            }
          },
          {
            "table": {
              "table_name": "products",
              "access_type": "ref",
              "possible_keys": [
                "PRIMARY",
                "status"
              ],
              "key": "status",
              "used_key_parts": [
                "status"
              ],
              "key_length": "3",
              "ref": [
                "const"
              ],
              "rows_examined_per_scan": 1420,
              "rows_produced_per_join": 1420,
              "filtered": "100.00",
              "cost_info": {
                "read_cost": "356.43",
                "eval_cost": "142.00",
                "prefix_cost": "498.43",
                "data_read_per_join": "6M"
              },
              "used_columns": [
                "product_id",
                "status",
                "amount",
                "usergroup_ids",
                "tracking",
                "stoc_intern",
                "pret_achizitie",
                "pret_revanzator",
                "gestiune"
              ],
              "attached_condition": "((0 <> (case `maniamall_cscart`.`products`.`tracking` when 'O' then <cache>((NULL > 0)) when 'B' then (`maniamall_cscart`.`products`.`amount` > 0) else 1 end)) and ((`maniamall_cscart`.`products`.`usergroup_ids` = '') or (0 <> find_in_set(0,`maniamall_cscart`.`products`.`usergroup_ids`)) or (0 <> find_in_set(1,`maniamall_cscart`.`products`.`usergroup_ids`))))"
            }
          },
          {
            "table": {
              "table_name": "prices",
              "access_type": "ref",
              "possible_keys": [
                "usergroup",
                "product_id",
                "lower_limit",
                "usergroup_id"
              ],
              "key": "lower_limit",
              "used_key_parts": [
                "lower_limit",
                "product_id"
              ],
              "key_length": "6",
              "ref": [
                "const",
                "maniamall_cscart.products.product_id"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 710,
              "filtered": "50.00",
              "using_index": true,
              "cost_info": {
                "read_cost": "451.82",
                "eval_cost": "71.00",
                "prefix_cost": "1092.25",
                "data_read_per_join": "16K"
              },
              "used_columns": [
                "product_id",
                "lower_limit",
                "usergroup_id"
              ],
              "attached_condition": "(`maniamall_cscart`.`prices`.`usergroup_id` in (0,0,1))"
            }
          },
          {
            "table": {
              "table_name": "cscart_product_sales",
              "access_type": "ref",
              "possible_keys": [
                "PRIMARY",
                "pa"
              ],
              "key": "pa",
              "used_key_parts": [
                "product_id"
              ],
              "key_length": "3",
              "ref": [
                "maniamall_cscart.products.product_id"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 236,
              "filtered": "33.33",
              "index_condition": "(`maniamall_cscart`.`cscart_product_sales`.`amount` >= 1)",
              "cost_info": {
                "read_cost": "177.50",
                "eval_cost": "23.66",
                "prefix_cost": "1340.75",
                "data_read_per_join": "3K"
              },
              "used_columns": [
                "category_id",
                "product_id",
                "amount"
              ]
            }
          },
          {
            "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": [
                "maniamall_cscart.cscart_product_sales.category_id"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 11,
              "filtered": "5.00",
              "cost_info": {
                "read_cost": "0.00",
                "eval_cost": "1.18",
                "prefix_cost": "1364.42",
                "data_read_per_join": "30K"
              },
              "used_columns": [
                "category_id",
                "company_id",
                "usergroup_ids",
                "status"
              ],
              "attached_condition": "((`maniamall_cscart`.`cscart_categories`.`company_id` = 1) and ((`maniamall_cscart`.`cscart_categories`.`usergroup_ids` = '') or (0 <> find_in_set(0,`maniamall_cscart`.`cscart_categories`.`usergroup_ids`)) or (0 <> find_in_set(1,`maniamall_cscart`.`cscart_categories`.`usergroup_ids`))) and (`maniamall_cscart`.`cscart_categories`.`status` in ('A','H')))"
            }
          },
          {
            "table": {
              "table_name": "products_categories",
              "access_type": "eq_ref",
              "possible_keys": [
                "PRIMARY",
                "pt"
              ],
              "key": "PRIMARY",
              "used_key_parts": [
                "category_id",
                "product_id"
              ],
              "key_length": "6",
              "ref": [
                "maniamall_cscart.cscart_product_sales.category_id",
                "maniamall_cscart.products.product_id"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 11,
              "filtered": "100.00",
              "using_index": true,
              "cost_info": {
                "read_cost": "0.00",
                "eval_cost": "1.18",
                "prefix_cost": "1365.60",
                "data_read_per_join": "189"
              },
              "used_columns": [
                "product_id",
                "category_id"
              ]
            }
          },
          {
            "table": {
              "table_name": "shared_descr",
              "access_type": "eq_ref",
              "possible_keys": [
                "PRIMARY",
                "product_id",
                "company_id"
              ],
              "key": "PRIMARY",
              "used_key_parts": [
                "product_id",
                "lang_code",
                "company_id"
              ],
              "key_length": "13",
              "ref": [
                "maniamall_cscart.products.product_id",
                "const",
                "const"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 11,
              "filtered": "100.00",
              "cost_info": {
                "read_cost": "0.00",
                "eval_cost": "1.18",
                "prefix_cost": "1366.79",
                "data_read_per_join": "45K"
              },
              "used_columns": [
                "product_id",
                "lang_code",
                "company_id",
                "product"
              ]
            }
          },
          {
            "table": {
              "table_name": "descr1",
              "access_type": "eq_ref",
              "possible_keys": [
                "PRIMARY",
                "product_id"
              ],
              "key": "PRIMARY",
              "used_key_parts": [
                "product_id",
                "lang_code"
              ],
              "key_length": "9",
              "ref": [
                "maniamall_cscart.products.product_id",
                "const"
              ],
              "rows_examined_per_scan": 1,
              "rows_produced_per_join": 11,
              "filtered": "100.00",
              "cost_info": {
                "read_cost": "0.00",
                "eval_cost": "1.18",
                "prefix_cost": "1367.97",
                "data_read_per_join": "45K"
              },
              "used_columns": [
                "product_id",
                "lang_code",
                "product"
              ]
            }
          }
        ]
      }
    }
  }
}

Result