GreatSQL 在SQL中使用 HINT 語法修改會(huì)話變量
在 GreatSQL 支持一種新的優(yōu)化Hint，名字叫SET_VAR，這個(gè)特性支持用戶在查詢語句里修改 GreatSQL 數(shù)據(jù)庫的一些會(huì)話變量，當(dāng)然修改只是對(duì)當(dāng)前查詢會(huì)話生效，不會(huì)影響到其他會(huì)話。
SET_VAR語法
SET_VAR這個(gè)hint用于臨時(shí)設(shè)置系統(tǒng)變量的會(huì)話值(在單個(gè)語句的持續(xù)時(shí)間內(nèi)有效)
SET_VAR的用法: SET_VAR(var_name=value)
var_name是被臨時(shí)修改的會(huì)話變量名，value是會(huì)話變量的取值
greatsql> SELECT @@unique_checks;SELECT /*+ SET_VAR(unique_checks=OFF) */ @@unique_checks;SELECT @@unique_checks;
SELECT /*+ SET_VAR(sort_buffer_size = 16M) */ name FROM people ORDER BY name;
INSERT /*+ SET_VAR(foreign_key_checks=OFF) */ INTO t2 VALUES(2);
SELECT /*+ SET_VAR(optimizer_switch = 'mrr_cost_based=off') */ 1;GreatSQL 8.0 之前的操作方法
在 GreatSQL 8.0 之前要對(duì)一個(gè)查詢進(jìn)行會(huì)話變量修改，需要怎么操作：
1.查詢之前的系統(tǒng)變量
greatsql> SELECT @@optimizer_switch;2.備份系統(tǒng)變量
greatsql> SET @old_optimizer_switch = @@optimizer_switch;3.設(shè)置新的變量
greatsql> SET optimizer_switch='index_merge=off';4.運(yùn)行查詢語句
greatsql> SELECT empno,ename,deptno from emp limit 1;5.恢復(fù)之前的系統(tǒng)變量
greatsql> SET optimizer_switch = @old_optimizer_switch;是不是有點(diǎn)繁瑣，現(xiàn)在我們使用SET_VAR這個(gè)新特性，很方便的就可以做這個(gè)操作了。
GreatSQL 8.0 的操作方法
greatsql>explain SELECT empno,ename,deptno FROM emp WHERE deptno=10 or ename='CLARK';
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+--------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+--------------------------------------------+
| 1 | SIMPLE | emp | NULL | index_merge | deptno,idx_ename | deptno,idx_ename | 5,63 | NULL | 4 | 100.00 | Using union(deptno,idx_ename); Using where |
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+--------------------------------------------+
1 row in set, 1 warning (0.01 sec)從執(zhí)行計(jì)劃上看，SQL語句使用了索引合并（type=index_merge），如果不想該sql使用索引合并，則可以通過SET_VAR進(jìn)行控制。
greatsql>explain SELECT /*+ SET_VAR(optimizer_switch='index_merge=off') */ empno,ename,deptno FROM emp WHERE deptno=10 or ename='CLARK';
+----+-------------+-------+------------+------+------------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+------------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | emp | NULL | ALL | deptno,idx_ename | NULL | NULL | NULL | 14 | 38.10 | Using where |
+----+-------------+-------+------------+------+------------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec這個(gè)新特性是不是很方便呢，之前由于優(yōu)化器的某些設(shè)置，少量sql語句選擇了錯(cuò)誤的執(zhí)行計(jì)劃，導(dǎo)致查詢語句性能低下，又不能隨意更改線上數(shù)據(jù)庫的變量，有了SET_VAR這個(gè)新特性,對(duì)于這種情況，可以考慮在查詢語句中使用set_var優(yōu)化這條語句。
我們知道，使用hash jion時(shí)，會(huì)使用到j(luò)oin buffer，join buffer的大小由join_buffer_size控制，其默認(rèn)值為256k，哈希連接不能使用超過此數(shù)量的內(nèi)存。當(dāng)哈希連接所需的內(nèi)存超過可用量時(shí)，GreatSQL將使用磁盤上的文件來處理此問題，使用到了磁盤文件，性能會(huì)下降，如果只想針對(duì)單條語句設(shè)置join buffer就可以使用SET_VAR。
CREATE TABLE t1 (c1 INT, c2 INT);
CREATE TABLE t2 (c1 INT, c2 INT);
CREATE TABLE t3 (c1 INT, c2 INT);分別對(duì)t1,t2,t3 插入100萬，200萬，300萬數(shù)據(jù)
greatsql> SET @@cte_max_recursion_depth = 99999999;
greatsql> INSERT INTO t1
WITH recursive t AS (
SELECT 1 AS c1 ,1 AS c2
UNION ALL
SELECT t.c1+1,t.c1*2
FROM t
WHERE t.c1 <1000000
)
SELECT * FROM t;
Query OK, 1000000 rows affected (10.63 sec)
Records: 1000000 Duplicates: 0 Warnings: 0
greatsql> SELECT @@join_buffer_size;
+--------------------+
| @@join_buffer_size |
+--------------------+
| 262144 |
+--------------------+
1 row in set (0.00 sec)
greatsql> SELECT * FROM t1
JOIN t2 ON (t1.c1 = t2.c1 AND t1.c2 < t2.c2)
JOIN t3 ON (t2.c1 = t3.c1);
Empty set (6.91 sec)
greatsql> SELECT /*+ SET_VAR(join_buffer_size=16777216) */ * FROM t1
JOIN t2 ON (t1.c1 = t2.c1 AND t1.c2 < t2.c2)
JOIN t3 ON (t2.c1 = t3.c1);
Empty set (5.87 sec)注意事項(xiàng)
1、并非所有會(huì)話變量都允許與SET_VAR一起使用。如果設(shè)置不支持用SET_VAR更改的系統(tǒng)變量，則會(huì)出現(xiàn)警告。
greatsql> SELECT /*+ SET_VAR(collation_server = 'utf8mb4') */ 1;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set, 1 warning (0.01 sec)
greatsql> SHOW WARNINGS\G
*************************** 1. row ***************************
Level: Warning
Code: 3637
Message: Variable 'collation_server' cannot be set using SET_VAR hint.
1 row in set (0.00 sec)2、SET_VAR語法只允許設(shè)置單個(gè)變量，但可以給出多個(gè)提示來設(shè)置多個(gè)變量:
SELECT /*+ SET_VAR(optimizer_switch = 'mrr_cost_based=off')
SET_VAR(max_heap_table_size = 1G) */ 1;3、如果沒有這個(gè)系統(tǒng)變量或變量值不正確，則忽略SET_VAR提示并發(fā)出警告
SELECT /*+ SET_VAR(max_size = 1G) */ 1;
SELECT /*+ SET_VAR(optimizer_switch = 'mrr_cost_based=yes') */ 1;第1條語句沒有 max_size 這個(gè)變量，語句2 的mrr_cost_based= on或者off, 企圖將其設(shè)置為 yes是錯(cuò)誤的，這兩個(gè)語句的 hint 都會(huì)被忽略，并產(chǎn)生一個(gè)warning。
greatsql> SELECT /*+ SET_VAR(max_size = 1G) */ 1;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set, 1 warning (0.00 sec)
greatsql> show warnings;
+---------+------+---------------------------------------------+
| Level | Code | Message |
+---------+------+---------------------------------------------+
| Warning | 3128 | Unresolved name 'max_size' for SET_VAR hint |
+---------+------+---------------------------------------------+
1 row in set (0.00 sec)
greatsql> SELECT /*+ SET_VAR(optimizer_switch = 'mrr_cost_based=yes') */ 1;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set, 1 warning (0.00 sec)
greatsql> show warnings;
+---------+------+-------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------------------------------------------+
| Warning | 1231 | Variable 'optimizer_switch' can't be set to the value of 'mrr_cost_based=yes' |
+---------+------+-------------------------------------------------------------------------------+
1 row in set (0.00 sec)4、SET_VAR提示只允許在語句級(jí)別使用。如果在子查詢中使用，則會(huì)被忽略并給出警告。
復(fù)制會(huì)忽略復(fù)制語句中的SET_VAR，以避免潛在的安全問題。
SET_VAR支持的變量
SET_VAR只是對(duì)部分變量可以用的，整理了GreatSQL主要支持的變量供參考：
- bulk_insert_buffer_size
- default_table_encryption
- default_tmp_storage_engine
- div_precision_increment
- end_markers_in_json
- eq_range_index_dive_limit
- foreign_key_checks
- group_concat_max_len
- 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
- optimizer_switch
- optimizer_trace_max_mem_size
- range_alloc_block_size
- read_buffer_size
- read_rnd_buffer_size
- secondary_engine_cost_threshold
- select_into_buffer_size
- select_into_disk_sync
- select_into_disk_sync_delay
- show_create_table_skip_secondary_engine
- sort_buffer_size
- sql_auto_is_null
- sql_big_selects
- sql_buffer_result
- sql_mode
- sql_require_primary_key
- sql_safe_updates
- sql_select_limit
- time_zone (≥ 8.0.17)
- timestamp
- tmp_table_size
- unique_checks
- updatable_views_with_limit
- use_secondary_engine
- windowing_use_high_precision
參考文檔
- https://dev.mysql.com/doc/refman/8.0/en/optimizer-hints.html#optimizer-hints-set-var
- https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html
- https://dev.mysql.com/doc/refman/8.0/en/hash-joins.html
