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：

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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`;

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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：

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-> 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)

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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）。

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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：

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1	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：

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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服务器之家网站的支持！
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原文链接：https://www.cnblogs.com/zhjh256/p/11705792.html