Mysql 8.0.18 hash join测试(推荐)
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2023-02-20 16:03:56
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 joi...
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
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