oracle天杀的，8.0.19版本居然又把hash join优化器提示废弃了，卧槽，再不信你了。
CREATE TABLE COLUMNS_hj as select * from information_schema.`COLUMNS`;
INSERT INTO COLUMNS_hj SELECT * FROM COLUMNS_hj; -- 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（相比mariadb的hash join实现BNLH join来说，要稳定的多）。
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。
目前hash join仅支持普通连接hash join，不支持反连接、半连接、外连接。
2019/11/15更新：各种场景的对比，也可以参考下https://mysqlserverteam.com/hash-join-in-mysql-8/
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