删除性能还是选择多个联接？

如何解决删除性能还是选择多个联接？

我正在尝试进行大规模删除。

我认为使用联接而不是子查询可以提高性能。

我想到了以下查询：

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

即使选择的等效项是瞬时的，即使没有删除任何行也要花费很长时间：

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

那是为什么？如何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：存在另一个使用select的尝试

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

解决方法

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)
;

并确保索引t1.a,t2.a,t2.b,t3.b。

此外，如果要删除表中的大多数数据，建议您导出那些将不被删除的记录并截断源表。之后，您可以导入导出的记录。

,

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

,

如果要删除表中的大量行，通常将要保留的行移动到另一个表中，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;