Hash Join
Hash Join 不需要任何索引來執行，并且在大多數情況下比當前的塊嵌套循環算法更有效。
下面通過實例代碼給大家介紹Mysql 8.0.18 hash join測試，具體內容如下所示：
CREATE TABLE COLUMNS_hj as select * from information_schema.`COLUMNS`;
INSERT INTO COLUMNS SELECT * FROM COLUMNS; -- 最后一次插入25萬行
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;
必須使用format=tree（8.0.16的新特性）才能查看hash join的執行計劃：
-> 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。
再來看BNL，先創建索引（分別優化了，再對比效果才公平）。
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)
大約4.5秒。可見hash join效果還是杠杠的。
不得不吐槽下mysql的優化器提示，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優化器提示還是很好用的，可用來設置語句級參數（oracle支持，mariadb記得也支持了的），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支持的變量列表：
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
總結
以上所述是小編給大家介紹的Mysql 8.0.18 hash join測試,希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回復大家的。在此也非常感謝大家對腳本之家網站的支持！
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