删除性能与选择多个连接？

Question

我正在尝试执行大规模删除。

我认为使用联接而不是子查询可以提高性能。

我想出了这个查询：

delete t1
    from table1 t1
    join table2 t2  on t1.a = t2.a
    join table3 t3  on t2.b = t3.b;

这需要很长时间，即使没有删除任何行，尽管选择等效项是瞬时的：

select *
    from table1 t1
    join table2 t2 on t1.a = t2.a
    join table3 t3 on t2.b = t3.b;

这是为什么呢？我怎样才能使我的第一个查询更快？

编辑：执行计划

mysql> explain delete t1 from table1 t1 join table2 t2 on t1.a = t2.a join table3 t3 on t2.b = t3.b;
+----+-------------+-------+------------+-------+--------------------------+----------+---------+----------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys            | key      | key_len | ref      | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+--------------------------+----------+---------+----------+------+----------+-------------+
|  1 | SIMPLE      | t2    | NULL       | index | PRIMARY                  | b        | 257     | NULL     |    1 |   100.00 | Using index |
|  1 | DELETE      | t1    | NULL       | ref   | a,FK2354764DB4B32        | a        | 8       | db.t2.a  |    1 |   100.00 | NULL        |
|  1 | SIMPLE      | t3    | NULL       | ALL   | NULL                     | NULL     | NULL    | NULL     | 5000 |    10.00 | Using where |
+----+-------------+-------+------------+-------+--------------------------+----------+---------+----------+------+----------+-------------+

edit2：另一个尝试选择存在

mysql> explain delete from table1 t1 where exists (select 1 from table2 t2 where t2.a = t1.a and exists (select 1 from table3 t3 where t3.b = t2.b));
+----+--------------------+-------+------------+--------+---------------+---------+---------+---------------+------+----------+-------------------------------------------------------------------+
| id | select_type        | table | partitions | type   | possible_keys | key     | key_len | ref           | rows | filtered | Extra                                                             |
+----+--------------------+-------+------------+--------+---------------+---------+---------+---------------+------+----------+-------------------------------------------------------------------+
|  1 | DELETE             | t1    | NULL       | ALL    | NULL          | NULL    | NULL    | NULL          | 10000|   100.00 | Using where                                                       |
|  2 | DEPENDENT SUBQUERY | t2    | NULL       | eq_ref | PRIMARY       | PRIMARY | 8       | db.t1.a       |    1 |   100.00 | NULL                                                              |
|  2 | DEPENDENT SUBQUERY | t3    | NULL       | ALL    | NULL          | NULL    | NULL    | NULL          | 5000 |    10.00 | Using where; FirstMatch(t2); Using join buffer (Block Nested Loop)|
+----+--------------------+-------+------------+--------+---------------+---------+---------+---------------+------+----------+-------------------------------------------------------------------+
3 rows in set, 3 warnings (0.00 sec)

谢谢

Answer 1

delete from table1 t1 
WHERE exists (
  select 1 from table2 t2 
  where t1.a = t2.a 
    and exists ( select 1 from table3 t3 where t2.b = t3.b)
;

并确保 t1.a, t2.a, t2.b, t3.b 已编入索引。

另外，如果你要删除表上的大部分数据，我建议你将那些不会被删除的记录导出并截断源表。之后就可以导入导出的记录了。

Answer 2

如果要删除表中的大量行，将要保留的行移到另一个表中，然后截断并重新加载原始表，通常会更快：

-- select the rows we want to keep into a new table
create table tmptable as 
select *
from table1 t1
where not exists (
    select 1
    from table2 t2
    inner join table3 t3 on t3.b = t2.b
    where t2.a = t1.a
);

-- empty the original table
truncate table table1;  -- !! back it up first !!

-- reload it
insert into table1 select * from tmptable;

-- done
drop table tmptable;

Answer 3

delete from t1
 where t1.a in (select distinct t2.a from t2 inner join t3 on t2.b = t3.b)