mysql - TokuDB 排序时间在 ASC 和 DESC 之间不同

Question

这是从 Tokutek 下载的 MariaDB + TokuDB 7.1 社区。如果这是正常行为，请接受我的无知，但我对排序结果有疑问。我在两个排序方向(升序和降序)之间的排序时遇到了巨大的时间差异:

SELECT sql_no_cache id, createts, deleted
FROM sort_test
WHERE createts > '2000098'
ORDER BY createts asc

+---------+----------+---------+
| id      | createts | deleted |
+---------+----------+---------+
| 1999999 |  2000099 |    NULL |
| 2000000 |  2000100 |    NULL |
+---------+----------+---------+
2 rows in set (0.00 sec)

SELECT sql_no_cache id, createts, deleted
FROM sort_test
WHERE createts > '2000098'
ORDER BY createts desc

+---------+----------+---------+
| id      | createts | deleted |
+---------+----------+---------+
| 2000000 |  2000100 |    NULL |
| 1999999 |  2000099 |    NULL |
+---------+----------+---------+
2 rows in set (0.55 sec)

下面我展示了我的简化测试用例。这是表格:

CREATE TABLE `sort_test` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `createts` int(11) DEFAULT NULL,
  `deleted` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_createts` (`createts`)
) ENGINE=TokuDB

在这里，我使用以下过程用 200 万行填充表:

delimiter ;;

drop procedure if exists sort_test_populate;;

create procedure sort_test_populate()
begin
DECLARE int_val INT DEFAULT 1;
myloop : LOOP
  if (int_val > 2000000) THEN
    LEAVE myloop;
  end if;
  insert into sort_test (id, createts) values (int_val, int_val+100);
  set int_val = int_val +1;
end loop;
end;;

call sort_test_populate();;
Query OK, 1 row affected (28 min 2.80 sec)

这里又是我的测试查询:

SELECT sql_no_cache id, createts, deleted
FROM sort_test
WHERE createts > '2000098'
ORDER BY createts asc

2 rows in set (0.00 sec)

SELECT sql_no_cache id, createts, deleted
FROM sort_test
WHERE createts > '2000098'
ORDER BY createts desc

2 rows in set (0.55 sec)

这是“扩展解释”的结果，两个查询的结果相同:

+------+-------------+-----------+-------+---------------+--------------+---------+------+------+----------+-------------+
| id   | select_type | table     | type  | possible_keys | key          | key_len | ref  | rows | filtered | Extra       |
+------+-------------+-----------+-------+---------------+--------------+---------+------+------+----------+-------------+
|    1 | SIMPLE      | sort_test | range | idx_createts  | idx_createts | 5       | NULL |    2 |   100.00 | Using where |
+------+-------------+-----------+-------+---------------+--------------+---------+------+------+----------+-------------+

请注意，这不是我正在使用的确切数据，因为太多而无法包含在此处。我只是想创建一些测试数据来演示问题。我的问题是 - 为什么它会这样以及如何使降序查询更快？

Answer 1

这是索引条件下推 (ICP) 的一个已知错误。解决方法是通过全局或在执行此查询的 session 中设置 optimizer_switch 来禁用 ICP。

mysql> SET optimizer_switch='index_condition_pushdown=off';

(完全公开，我是 Tokutek 的员工，TokuDB 的制造商)