SELECT 
  b.row_position 
FROM 
  (
    SELECT 
      product_id, 
      @r := @r + 1 as row_position 
    FROM 
      (
        SELECT 
          products.product_id 
        FROM 
          cscart_products as products 
          LEFT JOIN cscart_product_descriptions as descr1 ON descr1.product_id = products.product_id 
          AND descr1.lang_code = 'ja' 
          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 = 'ja' 
        WHERE 
          1 
          AND cscart_categories.category_id IN (3) 
          AND cscart_categories.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 IN ('A') 
          AND prices.usergroup_id IN (0, 0, 1) 
        GROUP BY 
          products.product_id 
        ORDER BY 
          descr1.product asc
      ) AS a
  ) AS b 
WHERE 
  b.product_id = 4

Query time 0.00101

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "b",
      "access_type": "ref",
      "possible_keys": [
        "<auto_key0>"
      ],
      "key": "<auto_key0>",
      "used_key_parts": [
        "product_id"
      ],
      "key_length": "3",
      "ref": [
        "const"
      ],
      "rows": 0,
      "filtered": 100,
      "materialized_from_subquery": {
        "using_temporary_table": true,
        "dependent": false,
        "cacheable": true,
        "query_block": {
          "select_id": 2,
          "table": {
            "table_name": "a",
            "access_type": "ALL",
            "rows": 3,
            "filtered": 100,
            "materialized_from_subquery": {
              "using_temporary_table": true,
              "dependent": false,
              "cacheable": true,
              "query_block": {
                "select_id": 3,
                "ordering_operation": {
                  "using_filesort": true,
                  "grouping_operation": {
                    "using_temporary_table": true,
                    "using_filesort": false,
                    "nested_loop": [
                      {
                        "table": {
                          "table_name": "shared_descr",
                          "access_type": "system",
                          "possible_keys": [
                            "PRIMARY",
                            "product_id",
                            "company_id"
                          ],
                          "rows": 0,
                          "filtered": 0,
                          "const_row_not_found": true
                        }
                      },
                      {
                        "table": {
                          "table_name": "cscart_categories",
                          "access_type": "const",
                          "possible_keys": [
                            "PRIMARY",
                            "c_status",
                            "p_category_id"
                          ],
                          "key": "PRIMARY",
                          "used_key_parts": [
                            "category_id"
                          ],
                          "key_length": "3",
                          "ref": [
                            "const"
                          ],
                          "rows": 1,
                          "filtered": 100
                        }
                      },
                      {
                        "table": {
                          "table_name": "products_categories",
                          "access_type": "ref",
                          "possible_keys": [
                            "PRIMARY",
                            "pt"
                          ],
                          "key": "PRIMARY",
                          "used_key_parts": [
                            "category_id"
                          ],
                          "key_length": "3",
                          "ref": [
                            "const"
                          ],
                          "rows": 3,
                          "filtered": 100,
                          "using_index": true
                        }
                      },
                      {
                        "table": {
                          "table_name": "products",
                          "access_type": "eq_ref",
                          "possible_keys": [
                            "PRIMARY",
                            "status"
                          ],
                          "key": "PRIMARY",
                          "used_key_parts": [
                            "product_id"
                          ],
                          "key_length": "3",
                          "ref": [
                            "_testcscart.products_categories.product_id"
                          ],
                          "rows": 1,
                          "filtered": 100,
                          "attached_condition": "(((`_testcscart`.`products`.`usergroup_ids` = '') or find_in_set(0,`_testcscart`.`products`.`usergroup_ids`) or find_in_set(1,`_testcscart`.`products`.`usergroup_ids`)) and (`_testcscart`.`products`.`status` = 'A'))"
                        }
                      },
                      {
                        "table": {
                          "table_name": "prices",
                          "access_type": "ref",
                          "possible_keys": [
                            "usergroup",
                            "product_id",
                            "lower_limit",
                            "usergroup_id"
                          ],
                          "key": "usergroup",
                          "used_key_parts": [
                            "product_id"
                          ],
                          "key_length": "3",
                          "ref": [
                            "_testcscart.products_categories.product_id"
                          ],
                          "rows": 1,
                          "filtered": 100,
                          "using_index": true,
                          "attached_condition": "((`_testcscart`.`prices`.`lower_limit` = 1) and (`_testcscart`.`prices`.`usergroup_id` in (0,0,1)))"
                        }
                      },
                      {
                        "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": [
                            "_testcscart.products_categories.product_id",
                            "const"
                          ],
                          "rows": 1,
                          "filtered": 100,
                          "attached_condition": "<if>(is_not_null_compl(descr1), (`_testcscart`.`descr1`.`lang_code` = 'ja'), true)"
                        }
                      }
                    ]
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}