Question

上周我安装了一些额外的数据库监控，后来发现我们数据库负载的30％花费在单个表上的一个查询上（目前有600万个）行）：

delete FROM mdl_grade_items_history WHERE timemodified < ?

在测试环境中，我尝试进行一些架构更改：

在此查询上运行EXPLAIN表示每次运行此查询时，都会进行全表扫描。

EXPLAIN DELETE FROM mdl_grade_items_history WHERE timemodified < '1490528405';
+----+-------------+-------------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table                   | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | DELETE      | mdl_grade_items_history | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 140784 |   100.00 | Using where |
+----+-------------+-------------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set (0.00 sec)

检查EXPLAIN（非常相似）SELECT查询会显示类似情况。

EXPLAIN SELECT id FROM mdl_grade_items_history WHERE timemodified < '1490528405';
+----+-------------+-------------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table                   | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | mdl_grade_items_history | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 140784 |    33.33 | Using where |
+----+-------------+-------------------------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

检查表定义，timemodified

上似乎没有索引

SHOW INDEX FROM mdl_grade_items_history;
+-------------------------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table                   | Non_unique | Key_name                | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------------------------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| mdl_grade_items_history |          0 | PRIMARY                 |            1 | id          | A         |      140784 |     NULL | NULL   |      | BTREE      |         |               |
| mdl_grade_items_history |          1 | mdl_graditemhist_act_ix |            1 | action      | A         |           2 |     NULL | NULL   |      | BTREE      |         |               |
| mdl_grade_items_history |          1 | mdl_graditemhist_old_ix |            1 | oldid       | A         |       17170 |     NULL | NULL   |      | BTREE      |         |               |
| mdl_grade_items_history |          1 | mdl_graditemhist_cou_ix |            1 | courseid    | A         |        1065 |     NULL | NULL   | YES  | BTREE      |         |               |
| mdl_grade_items_history |          1 | mdl_graditemhist_cat_ix |            1 | categoryid  | A         |        2300 |     NULL | NULL   | YES  | BTREE      |         |               |
| mdl_grade_items_history |          1 | mdl_graditemhist_sca_ix |            1 | scaleid     | A         |           6 |     NULL | NULL   | YES  | BTREE      |         |               |
| mdl_grade_items_history |          1 | mdl_graditemhist_out_ix |            1 | outcomeid   | A         |           1 |     NULL | NULL   | YES  | BTREE      |         |               |
| mdl_grade_items_history |          1 | mdl_graditemhist_log_ix |            1 | loggeduser  | A         |          30 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------------------------+------------+-------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
8 rows in set (0.00 sec)

所以我尝试添加一个（通过CREATE INDEX和ALTER TABLE .. ADD INDEX）

CREATE INDEX `mdl_gradeitemhist_tim_ix` ON `mdl_grade_items_history` (`timemodified`);
ALTER TABLE `mdl_grade_items_history` ADD INDEX `mdl_gradeitemhist_tim_ix` (`timemodified`);

在这两个实例中，SELECT查询都受到影响（请注意type）

中的更改

EXPLAIN `SELECT` id FROM mdl_grade_items_history WHERE timemodified < '1490528405';
+----+-------------+-------------------------+------------+-------+--------------------------+--------------------------+---------+------+-------+----------+--------------------------+
| id | select_type | table                   | partitions | type  | possible_keys            | key                      | key_len | ref  | rows  | filtered | Extra                    |
+----+-------------+-------------------------+------------+-------+--------------------------+--------------------------+---------+------+-------+----------+--------------------------+
|  1 | SIMPLE      | mdl_grade_items_history | NULL       | range | mdl_gradeitemhist_tim_ix | mdl_gradeitemhist_tim_ix | 9       | NULL | 70206 |   100.00 | Using where; Using index |
+----+-------------+-------------------------+------------+-------+--------------------------+--------------------------+---------+------+-------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

但不是DELETE查询。

EXPLAIN DELETE FROM mdl_grade_items_history WHERE timemodified < '1490528405';
+----+-------------+-------------------------+------------+------+--------------------------+------+---------+------+--------+----------+-------------+
| id | select_type | table                   | partitions | type | possible_keys            | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------------------------+------------+------+--------------------------+------+---------+------+--------+----------+-------------+
|  1 | DELETE      | mdl_grade_items_history | NULL       | ALL  | mdl_gradeitemhist_tim_ix | NULL | NULL    | NULL | 140412 |   100.00 | Using where |
+----+-------------+-------------------------+------------+------+--------------------------+------+---------+------+--------+----------+-------------+
1 row in set (0.00 sec)

我做错了什么？我还能尝试什么？

Answer 1

几乎从不使用低基数索引（action，scaleid，outcomeid）。摆脱它们。
拥有大量单列索引是一个红旗。请了解＆＃34;复合材料＆＃34;的功效和益处。索引。（与此处提到的选择/删除无关，但可能与其他查询相关。）
表上的额外索引会略微减慢INSERTs和DELETEs，因为索引需要（最终）更新。
如果修改了索引列，则额外索引会减慢UPDATEs。
CREATE INDEX和ALTER TABLE ADD INDEX做同样的事情;你现在可能有一个冗余索引。
EXPLAINs不同，因为（1）SELECT和DELETE做不同的事情，（2）EXPLAIN不是很复杂。
删除大量行需要花费很多精力 - 请记住，在ROLLBACK的情况下，已删除的行会挂起。只有在COMMIT之后才能真正删除这些行。（使用autocommit=ON时，隐含COMMIT。）

有关大删除的提示：

从mdl_grade_items_history优化DELETE

1 个答案: