Mysql 8.0.18 hash join测试(推荐)

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，如果大家有任何疑问请给我留言，小编会及时回复大家的。在此也非常感谢大家对共生网络网站的支持！

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