Mysql 8.0.18 hash join测试(推荐)

作者: 北方的狼72913907
来源: 51数据库
2021-10-07

hash join

hash join 不需要任何索引来执行，并且在大多数情况下比当前的块嵌套循环算法更有效。

下面通过实例代码给大家介绍mysql 8.0.18 hash join测试，具体内容如下所示：

create table columns_hj as select * from information_schema.`columns`;
insert into columns select * from columns; -- 最后一次插入25万行

create table columns_hj2 as select * from information_schema.`columns`;

explain format=tree
select 
 count(c1. privileges),
 sum(c1.ordinal_position)
from
 columns_hj c1,
 columns_hj2 c2
where
 c1.table_name = c2.table_name
and c1.column_name = c2.column_name
group by
 c1.table_name,
 c1.column_name
order by
 c1.table_name,
 c1.column_name;

必须使用format=tree（8.0.16的新特性）才能查看hash join的执行计划：

-> sort: <temporary>.table_name, <temporary>.column_name
 -> table scan on <temporary>
  -> aggregate using temporary table
   -> inner hash join (c1.`column_name` = c2.`column_name`), (c1.`table_name` = c2.`table_name`) (cost=134217298.97 rows=13421218)
    -> table scan on c1 (cost=1.60 rows=414619)
    -> hash
     -> table scan on c2 (cost=347.95 rows=3237)

set join_buffer_size=1048576000;

select 
 count(c1. privileges),
 sum(c1.ordinal_position)
from
 columns_hj c1,
 columns_hj2 c2
where
 c1.table_name = c2.table_name
and c1.column_name = c2.column_name
group by
 c1.table_name,
 c1.column_name
order by
 c1.table_name,
 c1.column_name;

1.5秒左右。

再来看bnl，先创建索引（分别优化了，再对比效果才公平）。

alter table columns_hj drop index idx_columns_hj;
alter table columns_hj2 drop index idx_columns_hj2;
create index idx_columns_hj on columns_hj(table_name,column_name);
create index idx_columns_hj2 on columns_hj2(table_name,column_name);

-> sort: <temporary>.table_name, <temporary>.column_name
 -> table scan on <temporary>
  -> aggregate using temporary table
   -> nested loop inner join (cost=454325.17 rows=412707)
    -> filter: ((c2.`table_name` is not null) and (c2.`column_name` is not null)) (cost=347.95 rows=3237)
     -> table scan on c2 (cost=347.95 rows=3237)
    -> index lookup on c1 using idx_columns_hj (table_name=c2.`table_name`, column_name=c2.`column_name`) (cost=127.50 rows=127)

大约4.5秒。可见hash join效果还是杠杠的。

不得不吐槽下mysql的优化器提示，貌似hash_join/no_hash_join都不生效。

除了hash_join外，mysql 8.0.3引入的set_var优化器提示还是很好用的，可用来设置语句级参数（oracle支持，mariadb记得也支持了的），如下：

mysql> select /*+ set_var(optimizer_switch='index_merge=off') set_var(join_buffer_size=4m) */ c_id from customer limit 1;

set_var支持的变量列表：

auto_increment_increment
auto_increment_offset
big_tables
bulk_insert_buffer_size
default_tmp_storage_engine
div_precision_increment
end_markers_in_json
eq_range_index_dive_limit
foreign_key_checks
group_concat_max_len
insert_id
internal_tmp_mem_storage_engine
join_buffer_size
lock_wait_timeout
max_error_count
max_execution_time
max_heap_table_size
max_join_size
max_length_for_sort_data
max_points_in_geometry
max_seeks_for_key
max_sort_length
optimizer_prune_level
optimizer_search_depth variables
optimizer_switch
range_alloc_block_size
range_optimizer_max_mem_size
read_buffer_size
read_rnd_buffer_size
sort_buffer_size
sql_auto_is_null
sql_big_selects
sql_buffer_result
sql_mode
sql_safe_updates
sql_select_limit
timestamp
tmp_table_size
updatable_views_with_limit
unique_checks
windowing_use_high_precision

总结

以上所述是小编给大家介绍的mysql 8.0.18 hash join测试,希望对大家有所帮助