mysql 8.0.18 hash join測試（內外網首文）

CREATE TABLE COLUMNS_hj as select * from information_schema.`COLUMNS`;
INSERT INTO COLUMNS SELECT * FROM COLUMNS;  -- 最後一次插入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，先建立索引（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效果仍是槓槓的。

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優化器提示仍是很好用的，可用來設置語句級參數（oracle支持，mariadb記得也支持了的），以下：

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